FX’s “Legion” pilot has found its leading man in Dan Stevens, Variety has learned.

The “Downton Abbey” and “The Guest” alum will star alongside Aubrey Plaza and Jean Smart in Noah Hawley’s Marvel project, and previously announced cast member Rachel Keller.

Production on the pilot is set to begin this March.

Stevens will star as David, a haunted man diagnosed as schizophrenic at a young age who’s trying to find his way back to sanity. He’s about to give up when he meets the girl of his dreams.

Plaza has been cast as David’s (Stevens) friend Lenny. Despite a life of drugs and alcohol abuse, she knows her life will soon turn around, which gives her the likeable energy of the impossible optimist despite her rough demeanor.

Smart will play Melanie, a nurturing, demanding therapist with a sharp mind and unconventional methods. “Legion” marks a reunion for Smart and Hawley, who worked together on the most recent season of “Fargo.”

Written by Hawley, “Legion,” based on the X-Men character who, in Marvel Comics canon, is Professor Charles Xavier’s mutant son.

Though the project is currently at the pilot phase, given Marvel’s success in the television landscape, plus Hawley’s significant relationship with FX as creator of their critically acclaimed “Fargo,” “Legion” is already a hot project for the cabler.

In addition to writing, Hawley will also serve as exec producer along with Lauren Shuler Donner, Bryan Singer, Simon Kinberg, Jeph Loeb, Jim Chory and John Cameron. The pilot is co-produced by FX Productions and Marvel TV with FX Prods. handling the physical production.

Up next, Stevens will star as the Beast in Disney’s live-action film adaptation of “Beauty and the Beast” alongside Emma Watson. Hawley is working on three more development projects at FX — “Cat’s Cradle,” “Hellhound on His Trail” and “The Hot Rock” — plus the upcoming season of “Fargo,” which was renewed for season three.