Look closely. They’re there. Really. Those teeny bugs — aphids, if you must be nit-picky — are everywhere in Tracy Letts’s feverishly pitched psychodrama. Or are they?

With sly humor and an intensity that is enthralling if often disconcerting, Boston Theatre Works taps deep recesses of the self-perpetuating paranoia of Letts’s new work, a follow-up to his Off Broadway hit Killer Joe. Although there are some slow moments in the first act, the second half more than compensates as it delves into cerebrally gripping subject matter — namely government conspiracies — at a rat-a-tat pace.

The play begins with Agnes, a middle-aged woman on the lam, sitting morosely, almost catatonically, on the edge of her bed in the dingy Oklahoma motel room where she lives. This prolonged moment establishes the loneliness that defines her life. Then her girlfriend R.C. (a spunky Kelly Lawman) drops by for some honky-tonk revelry. Relishing the company, Agnes tries to cheer herself by snorting cocaine, flirting with R.C., and nattering on about the regulars in the nightclub where she works. Then she homes in on R.C.’s companion, a shy yet jittery stranger who has an air of vulnerability and an interest in looking for hidden images in tacky paintings. Agnes invites him to stay, shrugging off his warning that " I make people nervous because I pick up on things. "

Those " things " are the substance of the heady and intricately wrought explanation Peter offers for his traumatic experience in the Army. A Gulf War vet, he is certain that the government implanted bugs in his blood. Now, as far as he’s concerned, he’s being trailed, and the bugs are flitting about the room. A self-made entomology expert by necessity, Peter soon makes Agnes his disciple. Before long they’re swatting at thin air, compulsively scratching their skin, and working out the nuances of Peter’s bleak conspiracy theory. But even as circumstances that erode the legitimacy of his fears come to light, Agnes has a genuine enemy to contend with. Her abusive ex-husband (a smarmy Trey Burvant) is out of prison on early parole, and he likes to stop by. It’s fascinating, in a pitiful way, to watch as Agnes and Peter build a bond as precarious as a house of cards. The line between terror and fantasy starts to blur as the bugs and their attendant dismal insinuations become the force that holds these two misfits together.

With this his second play, the Oklahoma-born Letts continues his grim exploration of marginal Midwesterners whose lives are constricted by the hardscrabble existence of the lower class to which they belong. Killer Joe, a winner at the Edinburgh Fringe Festival, was a disturbing look at a trailer-trash family and their shady exploits. Bug was scheduled to open Off Broadway last year, but in the wake of September 11 it was pulled and instead had its premiere at London’s Gate Theatre. Even now, after the nine high-anxiety months America has experienced, the play may hit some viewers a tad too close to home. Peter’s conviction that " you’re never really safe " resonates at several emotional decibels higher than Letts perhaps intended.

What faults Bug has lie more in the script than in this production. At one point, Letts briefly introduces a doctor from Peter’s past (a stilted Christopher J. Hagberg) whose main purpose is to supply exposition. When he then visits Agnes, their encounter feels contrived and the play’s rhythm is thrown.

For the most part, however, the play’s momentum is expertly handled by director Eric C. Engel, who plumbs the paranoia factor for all its nightmarish and comic possibilities. Those aspects are accentuated by Deb Sullivan’s alternately brash and eerie lighting and Susan Zeeman Rogers’s appropriately seedy motel-room set. The room’s zigzag walls appear primed to fold in on themselves, ensnaring those within and turning their fears into reality.

The lead actors’ performances render such fears palpable (and, if you’re the suggestible type, infectious). Adrianne Krstansky manifests Agnes’s codependent tendencies as she moves from false, chemical-induced confidence to genuine panic. And Augustus Kelley portrays Peter with a creepy boy-next-door quality, capturing the susceptibility that lends itself to extremism. His escalating physical outbursts are a sizzling sight — if, that is, you generally trust what you see.