Allie Kim's deathly allergy to sunlight, XP, still confines her to the night. Now that she's lost her best friend Juliet to an apparent suicide, the night has never felt darker—even with Rob at her side.

Allie knows why Juliet killed herself: to escape the clutches of Garrett Tabor, whom the trio saw committing an unspeakable crime. Garrett is untouchable; the Tabors founded the world-famous XP clinic that keeps Allie and Rob alive and their small Minnesota town on the map. Allie can't rest until Garrett is brought to justice, but her obsession jeopardizes everything she holds dear. Not even training for Parkour can distract her; nothing reminds her more that Juliet is gone. So, when Rob introduces Allie to the wildly dangerous sport of nighttime deep diving, Allie assumes he's only trying to derail her investigation—until they uncover the terrible secret Garrett Tabor has hidden under Lake Superior.