‘The Expanse’: Here’s a Recap of Seasons 1-3 Ahead of Season 4 on Amazon Prime – Space.com

Spoilers ahead.

Over the course of the last four years, "The Expanse" has proved, beyond any doubt, that it has earned its place among the very best of TV sci-fi, an impressive achievement that puts it alongside Ron Moore's "Battlestar Galactica" reboot, "Babylon 5," "Star Trek: The Original Series" and "Stargate SG1."

However, like "Babylon 5" and "Battlestar Galactica," "The Expanse" has a story arc structure rather than an episodic one and quite a complex story arc at that. Plus, it's been 18 months since we last saw the Rocinante spaceship roaring through space on our screens.

So, before you start watching the fourth season of "The Expanse," which drops on Amazon Prime today (Dec. 13), here's our handy guide to all the key events that took place in the first three seasons of the show.

Related: 'The Expanse' Seasons 1-3 Dock at Amazon Prime

Set a little over 300 years in the future, around 2350, humans have colonized space and three major factions have developed in the solar system: Earthers (governed by the United Nations), Martians and the Belters. The Earth itself has changed drastically and we see that sea levels have risen by 20 or 30 feet (6 to 9 meters), and the colony on Mars declared independence some time ago.

To quote Franklin Degraaf (Kenneth Welsh) from "Remember the Cant," Season 1 Episode 3: "You know what I love most about Mars? They still dream. We [Earthers] gave up. They're an entire culture dedicated to a common goal, working together as one to turn a lifeless rock into a garden. We had a garden and we paved it."

The Belters are a breakaway faction that has made the asteroid belt between Mars and Jupiter their home. One simple way of looking at this is that Earthers could be considered as the wealthy elite, Martians are the middle class and Belters are the working class. Conditions on the space stations and hollowed-out asteroid colonies are a long way from ideal, and the low gravity conditions to which they have adapted mean that most can never even visit Earth, because their bodies wouldn't be able to adjust to the increase in gravity. In the Belt, air and water are more precious than gold.

Then there's the Outer Planetary Alliance, or OPA, which started its life as a labor union or advocacy group, fighting for the interests of the Belters, and depending on who you ask, it's known as both a sociopolitical movement and a terrorist network.

Related: 'The Expanse' Season 4 Trailer Promises Exoplanet Action, Adventure and Intrigue

The complex story told in "The Expanse" begins with many separate elements that eventually intertwine. The first focuses on detective Josephus Miller (Thomas Jane), born on the dwarf planet Ceres in the asteroid belt, who is sent to find a missing young woman, Julie Mao (Florence Faivre). The second is in deep space, aboard the ice hauler Canterbury, which responds to a distress call from a ship called the Scopuli. Crew members Jim Holden (Steven Strait), Naomi Nagata (Dominique Tipper), Amos Burton (Wes Chatham) and Alex Kamal (Cas Anvar) head over to investigate in a shuttle called the Knight and find a planted distress beacon made with Mars tech, so they assume the Martians destroyed the Scopuli.

The Canterbury is then destroyed by unknown ships with stealth technology, and the Knight is rescued by the MCRN Donnager, the flagship of the Martian Congressional Republic Navy's Jupiter fleet. With tensions already at near-breaking point between the U.N. and the Martian government, the military forces of Mars are desperate to ensure collected sensor data about the mysterious stealth ships makes it back home. The Donnager is attacked by the same mysterious stealth ships. The four remaining Canterbury crewmembers narrowly escape to a frigate, called the Tachi, docked in the Donnager's landing bay. They rename their ship and change its transponder to the Rocinante to avoid attracting attention.

Eventually, they find safe harbor at Tycho station, the largest construction platform in the solar system. An enormous "generation ship" is being built here for the Mormons to travel to Tau Ceti, a journey that will take them around 100 years. The LDSS Nauvoo is just over 2 kilometers (1.2 miles) long and half a kilometer (0.3 miles) wide. Tycho station is also home to secret headquarters for the OPA, and the Rocinante crew forms a tentative alliance with its commanding officer, Fred Johnson (Chad Coleman Klyden from "The Orville").

They are sent by Johnson to investigate a derelict stealth ship called the Anubis, which is suspected of destroying the Scopuli and the Canterbury. Here they discover a strange, blue alien crystalized structure that's growing around the ship's reactor, which later becomes known as the protomolecule. They find that a shuttle from the Anubis had departed for Eros, so they nuke the ship and head to the asteroid.

Meanwhile, on Earth, U.N. Deputy Secretary Chrisjen Avasarala (Shohreh Aghdashloo) tries to avert an all-out war with Mars. Back in the Belt, Miller begins to get a little obsessed with Julie Mao, and with each new piece of evidence, he gets drawn deeper into a dark underworld where everyone from the OPA to his own police chief seems to have some sort of connection to her. The case leads him to Eros, where he meets and teams up with Holden and the crew of the Rocinante.

They find Julie Mao and learn that she was accidentally infected by the protomolecule via her rich, powerful, corporation-owning father, Jules-Pierre Mao (Franois Chau), who has been secretly working for the U.N. In fact, he has infected the whole population of Eros, and by faking a disaster, everyone there is forced into shelters, where they are unwittingly bombarded with radiation to encourage the protomolocule to consume them. The crew particularly Holden and Miller barely make it out alive.

Miller and the Rocinante crew make their way back to Tycho and discover that data from the Eros "disaster" is being transmitted to an unknown space station. They determine its location, mount a raid and kill all the scientists on board except one, who Fred Johnson takes back to Tycho station. Eros is deemed a biohazard, and a decision is made to destroy it by deliberately crashing the Nauvoo into it.

The Rocinante flies to Eros ahead of the Nauvoo to destroy the docking bays, thus ensuring that no scavengers board it to salvage any tech and risk accidental infection. Miller and a team of Belters land on the asteroid armed with explosives, but there's already a ship there. The Rocinante has no choice but to destroy it. However, the resulting debris damages Miller's bomb, and he's forced to stay behind to let the others safely escape. While he's waiting for the Nauvoo to collide, Eros suddenly changes course and starts accelerating toward Earth.

Because it is impossible for Eros to veer off course without some kind of external force acting upon it, alarm bells start ringing, and the U.N. launches a barrage of planet-busting nukes. Eros continues to accelerate toward Earth and then suddenly vanishes from radar. Miller starts thinking he can hear voices and see blue particles flying about. He follows them all the way back inside Eros, to the Blue Lagoon motel where Julie Mao's body was found, and theorizes that Eros is reacting to the Rocinante, which is following close behind and "painting" it with a laser for the nukes to zero in on.

The Rocinante backs off and Miller finds a protomolecule construct of Julie is "flying" Eros. He asks her to stop, but she says she can't, so he suggests instead of flying to Earth, perhaps she could fly toward Venus, which is uninhabited. The asteroid slams into Venus and the U.N. recovers most of its nuclear arsenal ... minus a couple that Fred Johnson is able to steal as "insurance." The Rocinante crew decide to destroy their sample of the protomolecule but Naomi, who originally wanted to give it to the OPA, instead hides it away in a missile that she leaves behind, floating in the asteroid belt, just in case.

Earth and Mars both send ships to study Venus. Meanwhile, there's an incident at an agricultural facility on Jupiter's moon Ganymede. Initially it seems it was just a flare up of tensions between Earth and Mars, but in reality it was a test of a protomolecule-human hybrid that attacks and kills all but one member of a Martian Marine Corp platoon, Gunnery Sergeant Bobbie Draper (Frankie Adams).

Fred Johnson and Holden have a falling out over the fate of the captured scientist, and then Anderson Dawes considered the father of the OPA and played deliciously by Jared Harris kidnaps him from under their noses and effectively kicks Fred Johnson out of the OPA.

Meanwhile, on Earth we learn that U.N. Undersecretary Sadavir Errinwright (Shawn Doyle) was working with Jules-Pierre Mao to develop the protomolocule, but they had a disagreement, so Mao instead offered it to the Martians. It turns out the Ganymede incident was a sales demonstration for the Martian Congressional Republic, but since they don't want Earth to know, they agree to a peace conference and blame Bobbie Draper for the whole incident. Bobbie realizes what's going on and defects from Mars to Earth.

On Tycho, some members of the OPA attempt to seize control of the station in order to use the nukes that Fred Johnson has against the "Inners" a derogatory term for both Martians and Earthers used by the "Outers." The insurrection is put down and some survivors from Ganymede arrive, including Dr. Praxidike "Prax" Meng (Terry Chen) a botanist who has lost his young daughter.

However, it turns out that his daughter wasn't killed on Ganymede. Rather, she was kidnapped by her pediatrician, who is also a scientist working for Jules-Pierre Mao and is in charge of the Hybrid program. The Rocinante crew head off to Ganymede, ostensibly to rescue Prax's daughter, but also because Holden wants to kill everyone involved with protomolocule program.

Avasarala and Bobbie arrange to meet with Jules-Pierre Mao on his private, luxury space yacht, in an attempt to persuade him to give Earth the protomolocule. However, Errinwright forces Mao to try and kill Avasarala, who was threatening to expose their connection and what really happened on Eros.

On Ganymede, Holden and Alex search for any remaining Hybrids, and Amos and Naiomi try to rescue as many refugees as they can. On Venus, both the Mars and Earth ships sent to further investigate the impact of Eros, are disintegrated into their component parts by the protomolocule as they approach the planet's surface.

Finally, we learn that before the Rocinante left for Ganymede, Naiomi believing they might die there told Fred Johnson of the location of the protomolocule sample that she secretly saved earlier which he then goes to recover. He sends his trusted officer Camina Drummer (Cara Gee) to salvage the Nauvoo for the Belters, feeling they needed a symbol-of-sorts now that they have the protomolocule and consequently a seat at the table, so to speak.

The third season picks up right where Season 2 ended. Bobbie, Avasarala and her bodyguard, Cotyar (Nick Tarabay), are pinned down by Jules-Pierre Mao's loyal men on the private yacht. Mao leaves and orders a U.N. ship to destroy the yacht, but Bobbie and Avasarala manage to escape in a racing pinnace that once belonged to Julie Mao and is still moored to the yacht. They have in their possession a recording of Errinwright confessing to everything. Unfortunately, this pinnace is a two-seater, so Cotyar must make his escape in the short-range travel pod that they initially arrived in.

Bobbie sends out a distress signal on a Martian-only frequency that's picked up by the Rccinante (formally a Martian frigate, don't forget), which rescues them. Errinwright dispatches U.N. Admiral Nguyen (Byron Mann) in the U.N. dreadnought, Agatha King, to fly to Io and collect the protomolocule from Jules-Pierre Mao. Along the way, it rescues Cotyar in the travel pod.

On the Rocinante, everyone is angry at Naiomi for lying about the protomolocule sample she saved and gave to Fred Johnson. Avasarala contacts U.N. Admiral Souther (Martin Roach), Nguyen's commanding officer, who's also on the U.N.N. Agatha King and sends him Errinwright's confession. They head to Io, where it's believed Prax's daughter is and therefore also the kidnapping pediatrician who heads the Hybrid program.

On Earth, Errinwright does his best to escalate tension between the U.N. and the Martian governments. He convinces the U.N. Secretary General, Esteban Sorrento-Gillis (Jonathan Whittaker), to launch a preemptive strike against Martian missile platforms and succeeds in destroying all but one, which gets off a retaliatory strike, nuking part of South America and killing millions. A trusted friend of the U.N. Secretary General, Anna Volovodov (Elizabeth Mitchell) confides in Avasarala over this act of war, who sends her the recording of Errinwright's confession in the hope Anna can persuade him to pacify the situation.

Both the Rocinante and the Agatha King head toward Jupiter's moon Io, where Jules-Pierre Mao has based his secret production facility. We find out that he's been using children to make the Hybrids, including Prax's daughter.

Onboard the Agatha King, Admiral Souther confirms Avasarala's story with Cotyar and tries to relieve Admiral Nguyen of command, but in the process he's shot and killed. The U.N. fleet is mostly loyal to Souther and refuses to follow Nguyen. There's a mutiny on board the Agatha King and Nguyen starts launching missiles loaded with Hybrids from Io in the direction of Mars. However, one of the missiles accidentally hits the Agatha King, infecting the ship.

The Rocinante crew land on Io to rescue the children and kill the evil, Hybrid-making scientists. Alex and Naomi board the Agatha King but can't stop the missiles en route to Mars. They plead with Fred Johnson to shoot them down and thus prevent an all-out war.

Cotyar sets the Agatha King to self destruct to prevent the protomolocule from spreading, and the Rocinante crew delivers Jules-Pierre Mao to Avasarala. The Secretary General arrests Errinwright, and on Venus, a strange structure rises up out of the crater that the Eros impact left behind and heads out into space, eventually taking up a stationary position near Uranus and forming a giant ring, or gate-like structure.

On Earth, Avasarala becomes the new Secretary General following Errinwight's arrest and the resignation of Sorrento-Gillis.

Vessels from Earth, Mars and the Belt rush to the new, giant alien ring, including the Nauvoo, now renamed the OPAS Behemoth and under the command of Cpt. Drummer with former space pirate Cmdr. Klaes Ashford (David Strathairn) serving as second-in-command and appointed by Anderson Dawes. Along the way, terrorists detonate a bomb onboard a U.N. ship and claim the ring belongs to the Belters, simultaneously framing Holden for the attack. Tension starts to mount, as everyone wants a piece of whatever the ring is. The only choice the crew of the Rocinante has at this point, is to run.

Holden starts to see visions of Miller, who instructs him to go through the gate but very slowly. Entering the gate too fast results in instantaneous and total deceleration, as one poor adolescent Belter discovers. The rest of the fleet follows the Rocinante into the gate, but they too are restricted by this "speed limit." Ahead of the Rocinante is a centralized object of some kind and the vision of Miller that Holden is seeing tells him to go EVA and further investigate this alien structure. The pursuing Martians launch a platoon of Marines, including Bobbie Draper, to intercept Holden.

Upon entering the structure, Holden determines with Miller's help that this alien "station" must be activated, but then the Marines arrive and the trigger-happy lieutenant wants to take Holden in, despite Bobbie's protests. The Marines fire a grenade and the station itself reacts to this hostile act by instantly stopping all movement inside the ring. The forward velocity of every single ship inside the ring is suddenly reduced to zero, causing a lot of injuries and fatalities for the U.N., the Martians and the Belter ships.

Holden and the remaining Marines return to their dropship, and Naomi who has been on the Behemoth as Chief Engineer returns to the Rocinante. All the wounded from the fleet are transported to the Behemoth, since that's the only vessel big enough to generate its own gravity.

The alien station meanwhile is powering up to destroy the solar system, and Cmdr. Ashford believes it should be destroyed. A vision of Miller tells Holden that this can be avoided if every single ship powers down their reactors to prove they're not hostile. A tense standoff between Ashford and the crew of the Rocinante results in several fire fights and very nearly the end of all life as we know it, but eventually the Behemoth joins every other ship in shutting down its reactor. So, the station shuts down it's "attack" and instead opens up the rest of the ring, which is indeed a gate, a gateway to thousands of other gateways, each one now offering access to a different part of the universe.

Following the mysterious construction and discovery of the "ring" a wormhole network that connects planetary systems across the Milky Way galaxy a new interplanetary "gold rush" has begun.

The U.N. Secretary General Chrisjen Avasarala charges the crew of the Rocinante to settle a land dispute on its new colony world New Terra. Meanwhile, the OPA begins transitioning the massive Behemoth spacecraft to become outpost and gatekeeper to humanity's new frontier.

Related: 'The Expanse' Season 4 Premieres This Week: Here's What to Expect.

Between the forces of both Mars and Earth looking to colonize the life-sustaining planet for their own means and undeniable evidence of the protomolecule's presence on the planet, it might just be anything but a potential new hope for displaced Belters. All the cast from Season 3 return, and they're joined by the brilliant Burn Gorman, who plays Adolphus Murtry.

Season 4 of "The Expanse" drops in its entirety on Amazon Prime today (Dec. 13).

Many thanks to Andy from the "Into The Expanse Podcast" for his kind assistance. If you want to take your fandom up a level, you can listen as he and his co-host Elton talk all things "Expanse" at Rogue Two Media.

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'The Expanse': Here's a Recap of Seasons 1-3 Ahead of Season 4 on Amazon Prime - Space.com