The architect Louis Kahn, who died of a heart attack in a Penn Station men’s room about 30 years ago, earned a towering reputation on the strength of just a handful of buildings from the ’60s and ’70s, including a pair of museums at Yale and the Salk Institute near San Diego. He was an inspiration, even a sort of guru, to architects who became much more famous among the general public, from Frank Gehry to I.M. Pei. And he was a terrible father—or at least an unreliable one. My Architect is an understated but stealthily powerful new documentary by Nathaniel Kahn, who is one of Lou’s three children, each by a different woman. (Nathaniel is pictured as a child, with Lou.) “Lou would visit every once in a while, usually at night,” recalls Nathaniel, who was 11 when his father died. “We never knew quite when it was going to be. He’d call at the last minute and say he was on his way. My mother would frantically whip up a five-course meal and have a martini in a frozen glass waiting for him.” The documentary alternates between such personal anecdotes and beautifully shot visits to Kahn’s masterworks, which alone among postwar landmarks found a way to combine the spare lines of modernist architecture with the power and gravity of classical ruins. By the film’s end, the two storylines run side by side: Nathaniel proves to be a surprisingly good analyst of the great warmth and humanism that his father put into his buildings, even as the architect gave his own children more erratic attention. My Architect screens at 4:15 p.m. Sunday, Dec. 14, in the National Gallery of Art’s East Building Auditorium, 4th Street and Constitution Avenue NW. Free. (202) 842-6799. (Christopher Hawthorne)