Catherine Chandler is a no-nonsense homicide detective who is haunted by a tragic past. In her youth, Cat witnessed the murder of her mother by two gunmen. She was almost killed as well, but someone – or something – came to her rescue. Since then, nobody has ever believed her story, but she knows it was not an animal that defended her from the assassins - it was a human.

While on a case, Cat and her partner Tess discover fingerprints that trace back to a doctor named Vincent Keller, who was reportedly killed while serving in the military. Knowing that he is actually alive, Cat digs deeper to discover that Vincent has been in hiding for the past 10 years, forced to live outside of society for unknown reasons. Vincent harbors a secret... when he gets enraged, he turns into a terrifying beast with uncontrollable super-strength and heightened senses.

Vincent admits to Cat that it was he who saved her the night of her mother's murder. But she doesn't know the whole truth - that Vincent has become emotionally attached to Cat after watching over her for years.