Terror stalks from beneath the Streets of Boston.

Detective Cait Reagan, forever changed by her relationship with Marcella Carson, has survived the opening of the Black Gate and the vampire curse. Now, seemingly cured, Cait has a new lease on her mortal life, but difficulties beset her at every turn. Cait's mother, Róisín, is neurologically devastated, unable to function without constant supervision, and her reunion with Morgan Kennedy, the first woman she ever loved, is downright awkward. In the meantime, Cait struggles to raise Katie, the sixteen-year-old vampire she turned in a desperate bid to save her life. Once Cait's enemy, Elizabeth is helping with Katie, but Cait's relationship with Liz is tenuous and strange. And eight-hundred-year-old vampire Nastasia Volkova, the acting head of the vampire council, seems to have her eye on Cait, refusing to remove her from the roll of known vampires.

As if all of this turmoil wasn't enough for one woman to handle, Marcella is missing, and someone is murdering the remaining vampires who survived the battle of the Black Gate. Nastasia has made it crystal clear that she wants the culprits found and brought to her brand of justice. Meanwhile, something sinister stalks from the Black Gate deep beneath Boston.

Cait must juggle her increasingly complicated personal life with the needs of her job, Nastasia's strange fixation, and the whirlwind of her own desperate search for self as she grasps for answers to the question, 'What is true evil?'