Threes, Half Cocked Productions, at the Space. Writer-director Arik Martin’s new play starts out like a tautly written episode of Law & Order, turns into something creepier and more treacherous midway through, and simply falls apart at the end. This is too bad, because Martin is a strong playwright and Half Cocked, which he cofounded, is known for urgent, harrowing theater.

Martin has a good ear, and when we meet his two detective characters–staid good cop (or is he?) Darrin Schiff (Todd Pate) and brash bad cop (or is he?) Paul Valentino (Jeremy Glickstein)–they talk just as we’d expect, in clipped sentences with lots of black humor. The two are investigating the rape of a fragile young woman, Abbie (the elfin Elizabeth Sidell), by someone in a skeleton costume on Halloween, but what she says happened doesn’t jibe with what they know. Later they confront Abbie’s protective older sister (a perfectly pitched Mary Kate Gallina) and odd brother (Gary Sugarman), have clever car conversations against a video backdrop of the streets they’re driving through, and interrogate suspects, including one nightmare-inducing felon played by Michael Rice.

Unfortunately there’s also a lot of pointless mystical talk about bad things happening in threes, and some twists fall in the murky valley between red herrings and actual plot points. A brutal final scene adds surprise but takes away much of the play’s earlier dark charm.