A doctor gets attacked by an intruder wielding a power tool. Lewis and Alves talk about his budding relationship. In Taos, N.M., a teacher and a doctor have been found dead with holes drilled through their skulls. A construction worker becomes the unsub's third victim. The unsub asks his victims, "Where is it?" The construction worker, Ben Montoya, had a gambling addiction. The unsub drilled through the victims' auditory cortex. The unsub, Caleb Sands, was refused building blueprints at the library. The unsub hears a humming sound when he gets upset. Caleb attacks Alma Hernandez, a library worker, in the parking lot, but stops when he realizes she is deaf. Lewis decides that the attacks have something to do with the Taos Hum, reports of which have been reported since the 1990s. The unsub is trying to find the source of the Hum. Caleb has created a map of potential sources. He abducts Jake Owens, an engineer at the power plant. Terry Rossett, who counsels local sufferers of the Hum, identifies Steve Conte, the first victim, who constructed "Hum-canceling boxes" and sold them worldwide. He added that Taos has an elevated electromagnetic field because of the way the power grid is set up. Caleb makes Owens turn off the power in District 1. The humming continues in Caleb's head, and he violently kills Owens with his power drill. Caleb remembers his wife Sarah, who was driven mad by the Hum. Garcia finds Sarah among a list of sufferers, then sends Caleb's photo, which matches the sketch they had. Sarah killed herself with a power drill, then Caleb, in his grief, inherited her affliction. Caleb grabs an axe and goes to the home he and Sarah once shared, now occupied by another couple. Lewis gets him to surrender.