The 100: Everything you need to know about season 4 

Catch all the best moments of the night  Learn More

After escaping space, Mount Weather, and murderous artificial intelligence, the Arkers of

are facing what started it all: nuclear apocalypse. In season 4 of CW’s sci-fi thriller, Earth is on the brink of collapse again, thanks to nuclear reactors melting down around the world. With roughly six months left before Polis falls to radiation, Clarke (Eliza Taylor) and company will have to find a solution. EW asked showrunner Jason Rothenberg for a head start to the end-of-days with this guide to everything you need to know before the launch of season 4.

“Season 4 starts right where season 3’s finale ended,” Rothenberg says. “Clarke has just pulled the lever, she has saved the day [and] destroyed A.L.I.E. (Erica Cerra) as her friends in the real world kept her alive long enough to do it.” Trouble is, she knows the world will end. Though Clarke tells Bellamy (Bob Morley) right away, the pair has to choose who else to trust with that knowledge to keep panic at bay.

But first, they’ll have to deal with a changing of the guard at Polis. “They realize pretty quickly that [telling people about the apocalypse] is going to have to wait because they have a five-minute problem,” Rothenberg says. Without a commander, different factions of Grounders will want to take the lead, with the Ice Nation proving even more formidable than before. It’ll be up to Clarke to figure out a way to save Roan (Zach McGowan), who survived the events of season 3 — but in the end, if the City of Light survivors and Grounders can’t find, well, common ground, surviving the apocalypse will have to wait.

In a word? Terrible. A.L.I.E., as misguided as she was, will turn out to be correct about the effects of rising radiation: Black rain will arrive first, water will become undrinkable, and pre-cancerous lesions will form on anyone exposed. “Those things all begin to happen,” Rothenberg warns, adding that every character will have to choose how to work with others when faced with near-certain death. “The story becomes man’s inhumanity to man, and man’s humanity to man… Can you transcend that? Will you let the fact that there’s less and less life-saving opportunities for the world drive you into a dark place and take everything you can for yourself and your people? Or will you figure out a way to link arms and become one people?”

Well first, Rothenberg makes it clear that Clarke won’t back down from the challenge: “Clarke’s a fighter. Clarke is an idea person, she’s always looking for a third way.”

Bellamy, meanwhile, will struggle with his guilt. He had a hand in Pike’s (Michael Beach) rise to power in season 3, played a part in fracturing his own people, and most importantly, thinks he had something to do with Octavia’s (Marie Avgeropoulos) decision to kill Pike and leave on her own. “That’s something that still haunts him, certainly, and now he’s given a second chance,” Rothenberg says. “He’s given an opportunity to do things differently… Part of Bellamy’s story is, at some point you have to let the people you love make their own mistakes. He’s been almost parental in the way that he took care of Octavia, and now he’s forced to, almost as a parent would, let her walk her own path.”

Speaking of which, Octavia will also have big questions to face before she contends with the end of the world. “She got revenge, but revenge and justice aren’t the same thing, and that’s something that she’s going to have to grapple with pretty seriously this season,” Rothenberg explains. “She’s not done going down a dark path, she’s still trying to fill a void left by what happened to Lincoln (Ricky Whittle) and trying to grapple with who she is now.” In other words, Octavia may no longer be the girl under the floor on the Ark, but she may also not be the warrior Indra (Adina Porter) wants her to be.

Eventually — and Rothenberg says this is barely a spoiler — news of the second Doomsday spreads, but while “nothing will stop Raven Reyes [Lindsey Morgan],” Jasper (Devon Bostick) will have a “surprising” reaction to the turn of events. In the original script for the season 3 finale, Rothenberg points out, Jasper was ready to end his own life — and that’s a thread that will get picked up again, but with a twist: “[The apocalypse] changes the math for how he wants to live the rest of his life.”

Jaha (Isaiah Washington), meanwhile, will have to contend with his guilt and his “sins,” as he calls them, over leading so many into the City of Light. The former chancellor will continue to try to save his people no matter what it takes because that’s a part of his DNA, Rothenberg says. Though he’ll be “in a dark place at the beginning of this season,” being out of power will help him be “reactivated” — even if Kane (Henry Ian Cusick) and Abby (Paige Turco) aren’t easily convinced. Still, there’s some good news, at least for shippers: When the season begins, Kane and Abby, Monty (Christopher Larkin) and Harper (Chelsey Reist), and John (Richard Harmon) and Emori (Luisa D’Oliveira) are going strong.

Finally, for more on new character Ilian (Chai Romruen), a Grounder who will butt heads with Clarke beginning in episode 2, go here.

Rothenberg has one last piece of advice for viewers: Because 

never likes to make things easy for anyone on screen,

 tragedy will take place this season. “The truth is this show has been, from day one, a show where characters die,” he says, adding that he and the writers 

think about how the audience will respond to character deaths, especially in the wake of season 3. “We’re not going to tell the story differently. If you are looking for a show where your characters are safe and they’re not going to die, this isn’t the show for you… Don’t be attached to anything. That’s the kind of world that it is and will always be.”

2 James Cameron defends \'Titanic\' ending: \'Mythbusters\' is \'full of sh--\'

3 See a bunch of Winona Ryder GIFs from the SAG Awards

5 Ashton Kutcher slams \'Muslim ban\' at SAG Awards