“How good is your memory?” is a Chilean private investigator’s go-to interview question with prospective agents, all of whom are in the 80-to-90 age range. Maite Alberdi’s charming documentary, a Sundance selection, is set in a Santiago nursing home and follows Sergio, the 83-year-old man hired by the investigator as a mole agent to scout out possible abuse inside the facility. What could otherwise have been a sad depiction of aging is humorous and poignant in the hands of Alberdi, who makes good-natured fun of old age—Sergio learns to record what he sees using a pair of 007-style glasses, for instance—while highlighting the wisdom we often forget comes with it. —J.V.