The BAU is called to investigate a series of deaths that appear completely natural—until evidence suggests a killer is using an undetectable toxin to commit the perfect murder. As the team hunts a brilliant toxicologist who leaves almost no trace behind, Y/N begins experiencing headaches, exhaustion, and strange lapses in concentration. Everyone assumes it's stress from the case. Including her. But when Reid discovers that her symptoms perfectly match those of the unsub's victims, the horrifying truth becomes clear: Y/N isn't just working the case. She's part of it. Now the BAU must race to catch a killer who has already claimed multiple lives—and find a way to save one of their own before it's too late.