The actual plot of The Expanse follows a number of plucky humans thrust into a cold war (which, in its third season, finally got pretty hot) between the three distinct powers, all vying for control and respect in their quadrant of the solar system. In Season 1, Belter James Holden (Steven Strait) and his crew are drawn into a massive conspiracy involving an extraterrestrial and deadly "protomolecule" after their ship answers a phony distress call, knocking down the first domino in nearly starting a three-way war. On Earth, expert political schemer Chrisjen Avasarala (Shohreh Aghdashloo) is steadily gaining control of the United Nations, the governing body of the entire planet, while in the Belt, detective Josephus Miller (Thomas Jane) is unraveling the mystery of who has been pulling the strings to incite a war in the first place. In its second season and beyond, part of the story follows headstrong Martian marine Bobbie Draper (Frankie Adams) as she loses faith with her home planet and throws in her lot with Holden and the crew of the Rocinante.
Like its view of humanity in our interstellar future, The Expanse is a little bit of everything: it's a bit of a space opera, a bit of a noir detective story, a bit of a western. Its view of the future is both bleak (i.e., it doesn't look like humans are going to stop warring with each other any time soon) and hopeful (if we can figure out how to colonize our corner of space, we can work together to find other habitable planets to start our lives on too). Without spoiling actual plot events for those of you who might want to go back and watch it all from the beginning (you should!), half of the fourth season, which follows the fourth book in the series, Cibola Burn, takes place on an Earth-like planet in a faraway solar system which two opposing parties, Earth and the Belt, are attempting to colonize, while the planet itself seems to have other plans.
As with a lot of hard sci-fi, the show is incrediblly complex and hard to parse (I've watched the third season in full twice over by now, and it still remains almost incomprehensible to me). But what's particularly impressive about The Expanse is, though the plots sometimes go all over the place, you'll reach a point where you suddenly realize you love these characters. Through simple osmosis, you'll learn what David Strathairn and Jared Harris mean when they launch into long monologues of Belter patois. You've gotta really pay attention to understand Avasarala's near-flawless rise to the top, but I guarantee by Season 2 you'll do anything for her. You've gotta listen well to appreciate Martian expat Alex Kamal's (Cas Anvar) poetic retellings of life out in the deep, dark black. But for those of us who are ready to enter into this world, the rewards of The Expanse, like the vast new frontier of outer space, are practically endless.