"Legion" follows the story of David Haller (played by Dan Stevens), a diagnosed schizophrenic who may actually be the most powerful mutant in the world. As David navigates his complex and troubled past, he discovers that he's been living in a mental institution for years, and that his memories may not be entirely his own. With the help of a group of fellow mutants, including Syd (Rachel Keller), Charles Xavier (Jemaine Clement), and Melanie Bird (Thandie Newton), David must uncover the truth about his past and his abilities.