William Keane (Damian Lewis) is that “crazy” man who comes to sit next to you on the bus. He is the “loony” muttering to himself on the deserted Tube platform late at night; he’s the “madman” raging down your high street, spewing foul words and scary promises. The Keanes of this world are embarrassing and frightening, so our natural inclination is to avert our eyes and protect our young. But the American director Lodge Kerrigan won’t let us look the other way.

Kerrigan is the great chronicler of mental health in America. His first film, Clean, Shaven (1994), was about a schizophrenic who goes to his home town in search of his daughter. Keane is also about a father who has lost a child, and himself. What’s more, he’s never absent from the screen. Kerrigan’s camera is so close that watching this film is like being on a bus and having that “lunatic” come and sit on your lap.

We first meet Keane in the New York Port Authority bus terminal, approaching staff and passers-by with a newspaper clipping. “Excuse me, have you seen this little girl?” he asks. We wonder: is this man desperate or deranged? We learn that, six months earlier, his daughter was abducted while with him at this very bus station. Later, we begin to wonder: did she really exist? Kerrigan and his handheld camera follow Keane as he wanders through a no man’s land of tunnels and motorways, dingy bars and fast-food joints. He keeps it raw and real. It’s a journey into paranoia and delusion, a battle with the twin demons of grief and guilt. “It’s your own fault — you let her go,” Keane tells himself.

The loss of his child has pushed him over the edge, but his condition is made worse by his social isolation. The film shows a man who has fallen through the safety net of social welfare and ended up in a kind of solitary confinement. Keane lives in a scuzzy hotel in New Jersey. He eats alone. He prowls the streets alone. He gulps down vodka and snorts cocaine on his own. Even when he has sex in the lavatory with a woman, it produces only a lonely spasm.

Keane’s descent into madness is halted when he meets Lynn (Amy Ryan), a single mum, and her seven-year-old daughter, Kira (Abigail Breslin, who played Olive in Little Miss Sunshine), who are staying in the same hotel. He lends them money; they lend him the possibility of reconnecting to family life. When Lynn asks him to look after Kira, our first reaction, despite all our liberal talk of understanding mental illness, is: lady, please don’t leave your daughter with the loony! The dramatic tension comes not from what we see, but from what we imagine will happen. But contact with the girl proves the best medicine: here we see Keane as the patient dad who will help with homework and makes sure Kira brushes her teeth.

What Kerrigan is showing us is the limitation of our stereotypes about the mentally ill. In press notes, he is quoted as saying: “In reality, people who suffer from mental illness are no more violent than anyone else.” Yet we see Keane beating up an innocent man he takes to be his child’s abductor.

Kerrigan’s films tend to rely on the big performance, and they don’t come much bigger than Lewis’s. It’s arresting and authentic. But the film is like watching a human car crash. We wallow in one man’s misery, but do we learn anything new? Not really. It’s a film the mental-health lobby will love, as will people who want so-called raw, vital cinema that shows the forgotten and displaced in society. This is social-worker cinema of the highest order.

Keane, 15, 94 mins, Three stars