Dexter is certain that Zach Hamilton killed his next door neighbor Cassie, who is found bludgeoned to death in her apartment. The scene suggests that the crime was impulsive rather than planned, as was the case with the death of Norma Rivera. Dexter finds Zach's blood under her fingernails but can't report it for obvious reasons. Zach is in the Keys - he's used his credit card - and Dexter and Hannah set off to find him. What he finds there impresses Dexter but also suggests that Zach isn't the killer. Meanwhile, Dexter is also helping Hannah acquire a new identity ...