Lily Yu is Rule Turner of Nokolai's Chosen. She is also an FBI Agent and is in Washington, D.C. taking a special version of training at nearby Quantico. She is part of the Secretive Unit Twelve with the Magical Crimes Division, and is currently on loan to the Secret Service. She is a Sensitive. The case she's been called in on is that of a Congressman offered a deal by a demon.

After a mysterious power surge of magic is felt the world over, fellow agent Cynna Weaver is summoned to help. While Cynna's gift is finding, she's also trained in spellcraft and demonology.

Sorcerer Cullen Seabourne also shows up complaining of having had his memory tampered with. He decides to help with this little "demon problem" the other agents are dealing with.

Randall, an heir to a southern werewolf clan is attacked and killed by a demon. The FBI agents of the MCD must band together to work this case and find the missing demon determined to kill - sent to kill. Lily, Rule, Cynna and Cullen must work together to find this demon and to save the world.

Blood Lines is the third book in this series. Tempting Danger and Mortal Danger are the first two and Blood Lines is better if read following the first two in the series.