Alright, I think I can do this without giving anything major away. The book opens with a teenage girl named Izie who is being sent to live at a private academy called Holbrook after a family tragedy. It's a place for kids with unusual backgrounds, and Izie struggles to fit in, haunted by what she's lost.
The 'thirteen' refers to her roommate, a girl named Oona, who is the thirteenth person to occupy that specific room. The school has its own strange rules and history, and Izie starts noticing that each of the previous twelve roommates left under mysterious circumstances, some vanishing completely. The synopsis is really about Izie trying to unravel the secret of the room and the school while navigating her own grief, all while forming a fragile bond with the enigmatic Oona. It's a gothic-tinged YA mystery that builds this incredible atmosphere of creeping dread; you're constantly questioning whether the threat is supernatural or something more human. The full arc deals with whether Izie can break the cycle or become the thirteenth victim herself.