What is most unsettling about "The Dreamers" is how much is unknown.

Something is putting people to sleep, possibly for good. Starting with a student on a college campus and quickly spreading to thousands of others, the unnamed syndrome's causes are not just unclear; they're barely even considered in Karen Thompson Walker's follow-up to "The Age of Miracles," which also dealt with an inexplicable disaster that suddenly shifted the way its characters viewed the world.

Walker is smart not to specify, knowing the setup will send readers' minds in many directions: Is climate change involved? A supervirus? Government conspiracy? Can we somehow blame it on Putin? In fact, one example of Walker's light touch, in a novel that could veer toward the heavy-handed, is that she avoids the sentiment it would be easy to wring out of a story in which many of the characters are fated to die. She presents even the most heartbreaking details — such as a single dad who always leaves a window open in case he succumbs, so that passers-by will be able to hear his infant sobbing — as if they're simply the facts of life in stressful times.

We get to know a few characters, including a pair of orphaned sisters and Mei, the shy roommate of the first victim. But, while Walker is ruthless in insisting that the disease strikes at random (meaning: no, being adorable will not save a newborn), she seems reluctant to make us invest too heavily in any of the novel's dozens of characters. As the title hints, "The Dreamers" is less about specific people than about a collective attempt to understand.

As in "Miracles," a coming of age novel in which the world suddenly slowed down, Walker is interested in how people respond in a crisis. Are they self-serving, or kind? Do they abandon their principles, or does hardship crystallize their humanity? How does it alter a life if you go to sleep not knowing if you'll ever wake up? Technically, of course, we all face that last question every single night, and one of this frightening book's tricks is to remind us to be grateful when we're given another day.

Walker writes beautifully about the things that define how a society either endures or collapses in crisis, a theme that may never have been more timely than it is now. That aspect of "The Dreamers" also may be what is most upsetting about it: the sense that we could decide to fix climate change, income inequity or any number of other ills Walker could be addressing with her central metaphor, but it would be too late. In fact, if we've been collectively asleep while all of these problems mounted, what would happen if we did wake up?

It wouldn't be pretty, according to one of the few dreamers who wakes.

"You've been unconscious for four days," a psychiatrist tells him, to which comes his devastating reply, "It's been a lot longer than that."

Chris Hewitt is a theater and film critic at the Star Tribune.

The Dreamers
By: Karen Thompson Walker.
Publisher: Random House, 303 pages, $27.