Set in early-2000s Salt Lake City, Mouse follows Denny, a lonely, 40-year-old low-life criminal and first generation Asian-American who lives with his mom, commits petty crimes for a living and has never quite found his place in this world. Denny's loneliness pushes him to sign up for a penpal service, where he meets Tess-a criminal in her own right. Tess and her gambling addict boyfriend, Maury, scam lonesome and unsuspecting people through a penpal grift they operate via newspaper classifieds. After Tess and Maury fall into debt with some dangerous people, they decide to venture to Salt Lake City to take Denny for all he's worth. Upon arrival, Tess and Maury discover they're not the only ones being untruthful.