Well, that escalated quickly. A couple of hits on the old crack pipe and quicker than you can say, "I think there might be insects nesting in my gums," television heartthrob James Norton has taken a pair of pliers to his molars.

Long before his epic family drama August: Osage County, Tracy Letts was the playwright laureate of trashy America. His debut Killer Joe (1993) was a trailer park thriller. Bug, which followed three years later, takes up residence in an Oklahoma motel with a divorced drug addict Agnes White (Kate Fleetwood). She's in hiding from her ex-husband Jerry (Alec Newman), just out of jail, ink still drying on his restraining order, when Norton's amiable Peter Evans knocks on her door like a knight in double denim. With his wet eyes and gentle features, he looks like he wouldn't hurt a fly...

Lying in bed one night, Peter's bitten by an insect - an aphid, he thinks - and, perfectly reasonably, starts scouring the sheets for more. Before long, the room's strewn with flypaper streamers and aerosol sprays, and Peter and Agnes can't stop scratching. But is this an infestation or just imagination? Peter's paranoid delusions - hatched during a stay in a military hospital - run away with the pair of them in a rash of self-inflicted sores the size of bullet holes.

It's a play that gets under your skin and gnaws away. Like Peter, it's deceptively easy-going to start, but, boy, does it work itself into a frenzy. Initially distracting, the claustrophobic confines of Found111 suit it perfectly. That we're packed close enough to catch our neighbour's nits only ups the itchy intensity of Simon Evans' production. Richard Howell's lighting adds an X Files glow, while Edward Lewis lets the outside world of coyotes and sirens and unidentifiable whirrings bleed through the thin motel walls.

But Evans also captures the play's tragic edge - the way two brittle people dig themselves deeper and deeper until they're beyond rescue. Norton becomes a silent scream of a man: slapping himself uncontrollably, blood leaking from his ear, almost trying to crawl out of his own skin. Fleetwood, gaunt as a ghost, brings back memories of her Lady Macbeth: a woman frazzled by fear. She clings to herself as if hyper-aware of every pore, apparently terrified of touching anything, and carries a pizza delivery box like a bomb that might blow.

Because what if the things that are meant to keep you safe turn out to be harmful themselves? Peter looks like the best thing that could possibly happen to Agnes, only to hollow her life out from the inside. Letts' play chews the idea over and over: husbands that hit their wives; fire alarms with radioactive components. Drugs might mute Agnes's anxiety, her grief for a vanished son, but they cause pretty huge problems of their own. And as for the government, the army, the FBI...

This is a fine study of paranoia - psychologically, philosophically and politically. Letts catches its wildfire nature, the way one tiny misgiving can unleash a plague, but shows that, far from being irrational, delusion escalates through sensible reasoning. Once Agnes is convinced of the infestation, it's searching for logical explanations that leads her to cuckoo conspiracy theories. Her reasoning's fine; her premises are false. That's what makes paranoia so inescapable. Everything is open to doubt - even friends trying to help you out - and the end result is a maelstrom of scepticism.

But if so, how do you trust anyone or anything - let alone the faceless powers in control of the country? Bug had currency 20 years ago, but it feels all the more potent in an age of dodgy dossiers and digital spying. It's still pulp fiction but it's got real purpose.

Bug runs at Found111 until 7 May.