Failure is one of the most potent American subjects, largely because of the drama implicit in our denial of it. This inspirational movie tells the true story of an unsuccessful salesman in San Francisco (Will Smith) who assumes custody of his young son as a single parent and contrives to switch professions. Smith is resourceful in the role, though the story stretches one’s credulity about his character’s resourcefulness. Gabrielle Muccino directed; with Thandie Newton and Jaden Smith (the star’s son). PG-13, 117 min.