A serial killer, with a fixation on John, is terrorizing Seattle. At each crime scene he leaves a pink envelope, marked with the symbol from John's scar. Inside each envelope there is a sympathy card, earning him the moniker The Mourner, containing a riddle that must be solved in order to find the next scene.

The team investigates the death of an up-and-coming local DJ. Meanwhile, Rachel re-enters John's life...and has some trouble adjusting.