The mug we follow is William Keane (Damian Lewis), wandering around a grimy bus terminal near Manhattan. He’s looking for his 7-year-old daughter. She was abducted in September, he explains, to the understandable surprise of passersby. We don’t know when September was, but we know it was long enough ago for those accosted to pick up their pace. Homeboy is certifiable.

As Keane tries to retrace the steps of his daughter’s kidnapper — getting on and off buses, taking a mid-day siesta in the median of a busy highway, scrubbing his genitals in a public restroom — Kerrigan and Lewis subtly convey the impression that this is all simply a normal day in the life of their man, a deftly performed series of scenes illustrating the kind of person who hangs out talking to himself.

Some moments come close to being overcooked, but Kerrigan and Lewis modulate Keane’s outbursts, and Kerrigan also knows what to do with extras, those background people who usually act like they’re trying not to notice what’s going on in the scene. Here they look like they notice what’s going on but are trying not to, just like any of us would if we walked in on a grown man rinsing his pits over a sink.

The shaky, hand-held camera used to capture all this has become by now one of the most clichéd grabs at a look of instability. A firsthand view of anyone, even a mentally disturbed person, will eventually become monotonous. The introduction of a surrogate daughter and her forlorn mom hints at a thickening of the plot, but not much comes of it. “Keane” seems content with showing what it’s like to be mentally ill. It is effective, but by the end the suspicion grows that Kerrigan has been swept up and lost in a technique. S