The set for Orphans, in boom! theater's freezing-cold South Lake Union warehouse, is arresting: A series of vertical wooden beams, with a framed rectangle in the middle, separates the audience from the stage. It's the fourth wall made literal, the frame a window—a trick that insulates the one-room world of the show, increasing its claustrophobia, but giving the audience a point of entry.

Phillip (Michael LaDell Harris) spends his entire life in this room. He's kept captive by his older brother, Treat (Ryan Brantley), for reasons both protective and sadistic. They're orphans in Philadelphia, and while Treat is out robbing people at knifepoint for cash and jewelry, Phillip is at home inhaling daytime television like The Price Is Right, memorizing brand names, and watching the world outside his window. He's a superb mimic and has secretly taught himself to read, hiding books under couch cushions when Treat comes home. We don't know exactly why, but it's important to Treat that his baby brother remain illiterate. When he finds an underlined word in their newspaper, he stalks Phillip around the house, accusing him of literacy.

Treat: You sure you don't have no pocket dictionary somewhere in this house? You sure you ain't spending the day reading the newspaper and books, underlining words, looking up the meaning of particular words, getting yourself an education?

Phillip: I got no education!

Treat: You know the alphabet?

Phillip: No!

Treat: I bet you know the fuckin' alphabet. I bet you're holding out on me.

Phillip: I ain't holding out on you, Treat.

Treat's total control over his brother is about to crack, and it's his own fault. Seeking ransom money, he kidnaps a drunk older man (James Patrick) he meets in a bar. When Treat leaves the house the following day, and the man wakes up tied to a chair with only Phillip for company, we know Treat has miscalculated. As the man, named Harold, slowly and effortlessly frees himself, Phillip warns him that Treat has a violent temper. "I love violent tempers," Harold purrs. He shares Treat's sociopathic mannerisms, with an even scarier level of darkness lurking under the surface. Treat is a vicious kid, but Harold is a real predator, preternaturally calm and almost affectless. When Treat comes home to his disaster, Harold has decided he likes these feral children. "I'm going to tame you," he says matter-of-factly. "I'm going to make you my very own." By the next scene, they're both under his spell.

These characters are fantastically complicated. (Lyle Kessler's script premiered in 1983 and has attracted a lot of famous names: It was directed by Gary Sinise at Steppenwolf in 1985, with John Mahoney as Harold; Alan J. Pakula directed a 1987 film version with Albert Finney in that role.) Both Brantley and Patrick are genuinely scary here, yet obviously have real affection for their captive charges. Harris's Phillip seems both developmentally delayed and a bit of a genius. Nicholas Spinarski's direction keeps the whole thing taut and creepy but still emotionally resonant, so we care for everyone on the stage even as they torment each other. This sick little family is seductively fascinating.