EXCLUSIVE: Iola Evans (Carnival Row), Adain Bradley (Riverdale) and Leo Howard (Why Women Kill) are the first to be cast in CW’s The 100 prequel backdoor pilot, from The 100 developer/executive producer/showrunner Jason Rothenberg.

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Written by Rothenberg, the prequel is being developed as a planted spinoff and will be introduced in an episode from the upcoming seventh and final season of The 100.

The Untitled 100 Prequel is set 97 years before the events of the original series. It starts with the end of the world — a nuclear apocalypse that wipes out most of the human population on Earth — and follows a band of survivors on the ground as they learn to cope in a dangerous world while fighting to create a new and better society from the ashes of what came before.

Evans will play Callie. Whip-smart and passionate with a rebellious streak to boot, Callie has spent years ignoring expectations in order to fight for the causes in which she believes. But when the apocalypse devastates the world she had been working so hard to save, Callie must find a new purpose, and future, for all that remains of humanity.

Bradley will portray Reese. Hard-working and headstrong, Reese has spent a lifetime in fierce competition with his beloved sister, Callie. But when the opportunity to finally prove his worth comes with an incredible cost, Reese must figure out what lines he is willing to cross to finally come out on top.

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Howard is August. A Rebellious and passionate musician, August is a member of a radical environmental group dedicated to saving the planet by any means necessary. After the apocalypse, he must figure out how to go on when everything he had been fighting for is gone.

Evans and Bradley have series-regular options to return if the pilot is picked up to series.

Rothenberg will executive produce with The 100 EP Leslie Morgenstein and Gina Girolamo of Alloy for The 100 producers, Alloy Entertainment in association with Warner Bros. TV and CBS Television Studios.

The 100 follows a group of post-apocalyptic survivors, largely a group of criminal adolescents. They are among the first people from a space habitat, the Ark, to return to Earth after a devastating nuclear apocalypse.

Evans trained at the Royal Conservatoire of Scotland. She just played the lead role in the National Theatre’s workshop for The Ocean at the End of the Lane, based on the Neil Gaiman novel. On television she appeared in a recent guest role on Amazon’s Carnival Row opposite Orlando Bloom and Cara Delavigne, and last year on ITV’s Vera, opposite Brenda Blethyn. She’s repped by Circle of Confusion and Waring and McKenna in the UK.

Bradley most recently wrapped the leading role in Constantin Film’s upcoming feature Wrong Turn: The Foundation and next will be seen in the indie feature Butter, alongside Mira Sorvino. His other TV credits include recurring roles on the CW’s Riverdale, Netflix’s All About the Washingtons and CBS’ The Bold and the Beautiful. He’s repped by Circle of Confusion, Gersh and Myman Greenspan.

Howard, a world-class martial artist, is known for incorporating his martial arts skills into his feature film and television roles, as Young Snake Eyes in G.I. Joe: The Rise of Cobra, as Young Conan in Conan the Barbarian and as Jack on the Disney XD comedy series Kickin’ It. He appears in CBS All Access series Why Women Kill and in a recurring role on the CW’s Legacies. He’s repped by Paradigm, Randy James Management and Buchwald.