What would you do if your 21-year-old son stopped returning your calls?

Maybe there's a new girl. Or a class with tons of homework. After all, it's easy to lose track of people in New York City. Most mothers would assume their son was just going through a phase.

Not Carol Meitzner.

She's the kind of mom who knows things, and the artist can always sense when something is really wrong with her beloved boy.

Jonas, for his part, has been sneaking around for awhile now. First he dropped out of NYU. Then there was that secret trip to Pakistan and a mysterious new friend, Masoud.

You guessed it: The question at the heart of "31 Hours," the riveting fourth novel by Masha Hamilton, is this: How would you react if someone you love fell in with potential terrorists?

"31 Hours" is part thriller, part love story, and it possesses the kind of velocity that makes it impossible to put down.

Told from many viewpoints, the book never succumbs to the disharmony that this sort of setup can create. Instead of throwing off the story's rhythm, the chorus of voices only adds to the forward motion.

Bottom line: Hamilton throws a lot of balls in the air. Lucky for readers, she's a decidedly talented juggler.

The book's unexpected scene-stealer is Sonny Hirt, a philosophical hobo who makes his living panhandling deep in the bowels of the Earth on the city's subway system. Sonny, like Carol, has an inexplicable ability to sense when trouble is ahead, and the urban soothsayer's visions add a delicious layer of tension.

Jonas' girlfriend, Vic, is the most multifaceted of this sterling cast, and she's devastated by his sudden disappearance. Assuming she's been dumped, she decides men are, "by their nature ... different from women. Men had this whole thing about the chase ... no matter how much they loved one woman, no matter how great she was, given a chance with another, they'd feel it sinful to say no, if they even for a second considered refusing. They deemed it their right -- their duty, maybe -- and the physical always took precedence over the emotional. Men thought as long as they were having sex, they couldn't be counted among the lonely, the aging, or the pathetic. Women knew otherwise."

This particular brand of realism -- whether justified or not -- underscores Hamilton's sotto voce message that even the simplest of human misunderstandings can lead to life-changing results.

While the author raises far more questions than she answers, "31 Hours" delivers a potent psychological analysis on the true meaning of loyalty -- to friends, family members and country -- and what any of us, given the chance, would to do to uphold it.

Andrea Hoag is a book critic in Lawrence, Kan.