When archivist Perpetua Maria Voss finds a cedar box hidden behind the wrong shelf at Chatsworth, she expects a cataloguing error. What she finds instead is a three-hundred-year-old warning, encoded in the pages of a real 1666 novel by Margaret Cavendish, Duchess of Newcastle -- and a hollow in the velvet exactly the shape of a vanished vial.

The trail leads through Cambridge archives, a sealed room beneath a working physics laboratory, and the quiet, unbroken record of keepers who have guarded a dangerous discovery since 1667 -- never using it, never destroying it, only waiting. When someone else finally decides three and a half centuries of restraint was cowardice rather than wisdom, Perpetua and the unlikely circle she's gathered must decide what four people --- and, perhaps, the very first oversight body ever built by consensus rather than secrecy --- owe to a truth that has outlived everyone who ever tried to carry it alone.

Elsewhere

The room held its shape a moment longer, unwindowed, unmeasured, two figures neither quite seated nor quite standing, and then, without any visible mechanism for the transition, it was empty, and somewhere very far from it, in a small pizza restaurant that had never once needed to know how close it sat to any of this, a woman put her phone away and chose, for one more evening, to let the people she loved have the whole of her attention instead.

Perhaps the most intriguing mystery ever. ...

"I've been keeping a different list," Kier said, when Perpetua finished. She turned to the whiteboard. "Thirty Nobel Prizes have come out of this department since it opened. That's a real number. It's also, statistically, a strange number, and everyone who works here treats it as flattering rather than strange, which is exactly the mistake I'd expect people to make about their own good fortune."