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Caution: potential spoilers.
First of all, I want to say that it's a bit hard to classify this series. It has a mildly sci-fi-ish aspect, but it appears to be essentially set in the present, and it's not super-implausible, I guess. I'd like to have been able to call it a limited series, but the 13-episode first season ends on a cliffhanger. So clearly the writers are hoping for a second season. I'm not sure if they'll get one or not, but either way, the story is not finished at this point. (If it does get a second season, I'd probably watch it, but if it doesn't, I won't be terribly disappointed.)
So, there's this MIT grad student named Liam Cole, who's been making a map of the solar system, or whatever. Or his computer is making a map. Or something. Anyway, one night when he's out at a bar, he meets a woman named Jillian Hayes, and the two of them go back to her place to hook up. While he's there, Liam's phone alerts him that his computer program has detected an asteroid headed for Earth, which would get here in like 186 days, and could cause an extinction-level event. He immediately goes to tell his professor, Dr. Malcolm Croft, who has friends in Washington, apparently. The next day, Croft doesn't show up to teach his class, so Liam goes back to the professor's house, and it looks as if Croft had been kidnapped or something. And when Liam calls 911 to report it, he spots a suspicious looking black SUV parked nearby, which ends up chasing him. He eludes it, and later goes to see a famous tech billionaire named Darius Tanz (whom we'd previously seen give a lecture that both Liam and Jillian had attended, before the two of them actually met). Liam shows Darius his data, which convinces him of the danger. So he takes Liam with him to D.C., to talk to Deputy Secretary of Defense Harris Edwards.
Before that meeting, we had seen that Harris is in a secret relationship with a member of his staff, Pentagon Press Secretary Grace Barrows (Jennifer Finnigan). Grace is divorced, and has a daughter named Zoe who is just graduating high school, and planning to go to college. However, Zoe surprises her mother with the information that she's planning to take a year off before college, to do volunteer work in Africa. (It seemed to me that we didn't see Zoe again for a few episodes, so I assumed she had already left, but she does show up later, because she hadn't left yet.) Also, when Darius and Liam show up at Harris's office to tell him about the asteroid, he takes both them and Grace to a secret room in the Pentagon where people have been tracking the asteroid for like six months, and working on plans to deal with it. It's top secret, of course, but now that Darius and Liam know about it, Harris decides to tell Grace about it, so she can help keep the news from getting out to anyone else. However, she soon finds out he's not telling her the whole truth. Meanwhile, Darius has his own plan to save the human race from extinction. He doesn't believe the Department of Defense's plan is going to work; instead, he wants to build an ark ship that could take a select group of 160 people to colonize Mars. He tells Liam and Grace about this, but neither of them want any part of it, at first. Liam goes back to Boston to see Jillian again, and... well, in what I consider a nice touch, she's a science fiction writer, and he asks for advice about his situation (without really letting her know it's a real situation), and that leads him to return to D.C. to help Darius, after all. And after Grace learns that Harris is still keeping secrets, she also goes back to Darius... to convince him to help the DOD make its plan work. Oh, and it turns out Darius had already secretly built his ark ship, which is called Salvation. And he eventually hires Jillian to be part of a group of people he has trying to select the 160 people to be saved from extinction by going on his ark ship (though it's just a theoretical thing to them, since the people debating which people from all of humanity are most worthy of saving have no idea the ship actually exists, nor that the Earth is in imminent danger). Anyway, Jillian's new job also means she gets to spend more time with Liam (though their relationship has its ups and downs).
There's also a reporter named Amanda Neel, who has taken an interest in Liam, as a possible source of information on whatever's going on with Darius. And she ends up uncovering several major secrets that the government has been keeping. And um... lots of stuff happens over the course of the season. It turns out there's a conspiracy against the U.S. President, Pauline Mackenzie. I don't want to say too much about that, but I do want to mention that one of the conspirators is the Vice President, which I only mention because he's played by Sasha Roiz, whom I know from Grimm. Other characters I want to mention mainly because the actors were familiar to me include Darius's childhood sweetheart, Theresa (aka "Tess," after whom he had named his company's A.I.), who comes back into his life following the death of her brother, who was the head of Tanz Industries' security. (Theresa is played by Autumn Reeser.) And Darius has an estranged uncle named Nicholas (John Noble). Some characters who are a bit more important to the story include Grace's father, a former CIA agent named Hugh Keating (Mark Moses, probably best known to me from Desperate Housewives); and a woman named Claire Rayburn, who works with Harris; and a young man named Dylan, who is Harris's son, and turns out to be involved in a hacktivist group called "Resist."
So anyway, there's lots of stuff going on, and it just gets more and more complicated, and the story seems to go in several directions (though they're all interconnected to some degree, I suppose). It can be a bit hard to follow at times, but mostly I found it interesting. And it's always nice to see familiar actors (though the main reason I watched was for Jennifer Finnigan, whom I've always liked but have seen in far too few things). And... I guess I don't want to spoil any of the plot developments.