Weakness follows the life of high school teacher Joshua Polansky (Bobby Cannavale), whose world is falling apart: his wife, Elizabeth (June Diane Raphael), wants to start swinging; his mother, Lenore (Phyllis Somerville), thinks she has terminal cancer; and his autistic brother, Pete (Keith Nobbs), who resides at an assisted living home. At school, things get complicated as a love triangle of sorts develops between Joshua and his teaching colleagues, the wise-cracking Bart (Josh Charles), and idealist Katharine (Lily Rabe). When Elizabeth leaves him, Joshua retreats from his responsibilities and begins dating Danielle, (Danielle Panabaker), a former student who has postponed attending college. Joshua's subsequent regression and indecision fail to mask his anger and hurt, leading to a series of events that force him to reconsider not only his decisions, but the subtle and profound ways that people affect one another.