McClelland & Stewart, 323 pp, $32.99, hardcover.

In the late '80s, American neurologist Oliver Sacks wrote an article in the New York Review of Books suggesting that the deaf inhabit a distinct culture that arises from having their own language. It was a startling idea at the time. Now, in her fictional Claire's Head, Catherine Bush seems to be exploring the idea that perhaps migraine sufferers belong to a distinct culture as well. The pain that Claire and her older sister, Rachel, experience affects their perceptions, their relationships, where they work, what they eat, drink, and think about. Migraines bind them tighter than family.

Sometimes a novelist discovers what she was writing about after the fact. Bush found herself proofreading her 1993 first novel (Minus Time) for the paperback edition while copy-editing her second (2000's Rules of Engagement), and she realized that both books dealt with the twin themes of risk and safety. Claire's Head, her third, revisits those themes in a different guise: is it better to circumscribe one's life to minimize pain or to engage with the world and accept that pain is a consequence?

In Claire's Head, Bush has tried to overlay her thematic exploration with a veneer of mystery. The novel opens with Claire discovering that her sister has gone missing, which sets Claire on a quest to find her that takes up the remainder of the novel. But the mix doesn't quite work. The mystery lacks the necessities of the genre: plot twists, surprises, dialogue that dances or delights. The scaffolding of the story takes too long to build.

With the discovery of Rachel's "headache diary" late in the book, the novel finally finds language as intense as the pain the sisters feel, and Claire, unmoored from the safety of home by her mission to find Rachel, is transformed into a character we can care about. From that point, Claire's Head develops an urgency that is inescapable.