Keir Gilchrist as Sam Gardner, an 18-year-old boy on the autism spectrum,[1][2] who is obsessed with Antarctica.[3] Gilchrist said about the character, "He's one person that is on the autism spectrum. He's a very specific character".[4] Brigette Lundy-Paine as Casey Gardner,[2] Sam's younger sister who is very protective of him [5] Jennifer Jason Leigh as Elsa Gardner,[2] Sam and Casey's overprotective mother. Michael Rapaport as Doug Gardner,[2] Sam and Casey's father and Elsa's husband Amy Okuda as Julia Sasaki,[2] Sam's therapist and love interest