Downton Abbey heartthrob Dan Stevens is returning to television as a superhero, signing on for the title role in Legion, FX’s X-Men-universe pilot from Fargo creator Noah Hawley. Co-starring in the pilot, from Marvel TV and FX Prods., are Aubrey Plaza and Jean Smart, joining previously cast Rachel Keller.

legionLegion introduces the story of David Haller (Stevens), based on the Marvel comics character Legion, the son of X-Men founder Charles Xavier. Since he was a teenager, David has struggled with mental illness. Diagnosed as schizophrenic, he has been in and out of psychiatric hospitals for years. A haunted man, David is getting tired of finding his way back to sanity and is about to give up when he meets the girl of his dreams, a fellow patient. After the strange encounter, he’s confronted with the possibility that the voices he hears and the visions he sees might be real.

Plaza will play David’s friend Lenny, Aubrey Plaza Jean Smartwho, despite a life of drugs and alcohol abuse, knows that any day now her life is gonna turn around, which gives her the likable energy of the impossible optimist despite her rough demeanor. Smart is Melanie, a nurturing, demanding therapist with a sharp mind and unconventional methods. Keller plays the self-sufficient and street-smart Syd, who uses her sharp and prickly demeanor to protect her soft core, because even though it makes her a sucker and puts her at risk, she still believes in happily ever after.

Fargo‘s Hawley wrote the pilot and serves as an executive producer along with Lauren Shuler Donner, Bryan Singer and Simon Kinberg of the X-Men feature franchise, Marvel TV’s Jeph Loeb, as well as Jim Chory and John Cameron.

Stevens shot to fame as Matthew Crawley on the hit British period drama Downton Abbey. Since departing the show after Season 3, he has been focused on features including The Guest and Night At The Museum: Secret Of The Tomb, and will next be seen as the Beast in the Disney live-action Beauty And The Beast opposite Emma Watson. Stevens, represented by WME, Julian Belfrage Associates and Peikoff/Mahan, joins former Downton co-star Michelle Dockery, who also toplines a U.S. basic cable drama, TNT’s Good Behavior.

Parks & Recreation alumna Plaza will next be seen in Mike And Dave Need Wedding Dates. She is repped by CAA, MGMT Entertainment and Ira Schreck.

Like Keller, Emmy winner Smart recently worked with Hawley on the second installment of FX’s Fargo. Smart, who won a Critics’ Choice Award for her role, will next co-star alongside Ben Affleck in director Gavin O’Connor’s The Accountant.

Keller is repped by WME and Suskin Management.