The 'Downton Abbey,' 'Parks and Recreation' and 'Fargo' alums join previously cast Rachel Keller (also from 'Fargo') in the Noah Hawley drama pilot.

FX is going big for its X-Men take, Legion.

The network has tapped Downton Abbey's Dan Stevens, Parks and Recreation grad Aubrey Plaza and Fargo alum Jean Smart to star in the pilot from Noah Hawley (Fargo), The Hollywood Reporter has learned.

The Legion pilot introduces the story of David Haller, a man who may be more than human and who has struggled with mental illness since his teenage years. Diagnosed as schizophrenic, David has been in and out of psychiatric hospitals for years. But after a strange encounter with a fellow patient, he is confronted with the possibility that the voices he hears and the visions he sees might be real. In the comics, David is the son of X-Men founder Charles Xavier (aka Professor X) and Israeli Holocaust survivor Gabrielle Haller. The character first appeared in New Mutants No. 25 in March 1985.

Stevens will play David, a haunted man trying to find his way back to sanity; he's getting tired and is about to give up when he meets the girl of his dreams.

For Stevens, the role marks his TV follow-up to Downton Abbey, where he played Matthew Crawley for three years (and 25 episodes). He most recently starred as the Beast in the live-action Beauty and the Beast movie. He's repped by WME, Julian Belfrage Associates and Peikoff Mahan.

Parks breakout Plaza will play Lenny, David's friend who, despite a life of drugs and alcohol abuse, knows that any day now her life is going to turn around — which gives Lenny the likeable energy of the impossible optimist despite her rough demeanor.

Since wrapping NBC's Parks, UCB grad Plaza — who has received multiple offers this pilot season — has focused on film with features Dirty Grandpa, Mike and Dave Need Wedding Dates and Joshy. She's repped by CAA, MGMT Entertainment and Schreck Rose.

Smart, who reunites with Hawley after her role in season two of FX's Fargo, is set as Melanie, a nurturing, demanding therapist with a sharp mind and unconventional methods. The actress' role in Fargo earned her a Critics' Choice Award for supporting actress in a limited series. The three-time Emmy winner next stars in The Accountant. She's repped by WME and Untitled Entertainment.

Stevens and Plaza join the previously cast Rachel Keller, a Fargo season two breakout who will also reunite with Hawley on Legion. Keller will play the female lead, Syd, a self-sufficient and street smart woman 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.

Legion hails from FX Productions and Marvel Television, with FXP overseeing production. Hawley, who has an overall with FXP, will write the pilot and serve as an executive producer alongside Lauren Shuler Donner (X-Men: Days of Future Past, Wolverine), Bryan Singer (X-Men: Days of Future Past, Superman Returns), Simon Kinberg (X-Men: Days of Future Past, The Martian) and Marvel's Jeph Loeb (Agents of SHIELD, Daredevil) and Jim Chory (Daredevil). John Cameron is also on board as an exec producer via his overall deal with FXP, marking his and Hawley's latest collaboration after Golden Globe-winning anthology Fargo. Production on Legion will begin in March.