When a black cat shows up on her back porch in the pouring rain, a young Isadora knows she has a new best friend. Something about him feels off, though. For instance, he refuses to answer to anything other than Sebastian and is better at math than she is! Coming from a long line of witches, her mother assures her that he’s her familiar, but Isadora swears the cat rolls his eyes every time she makes that comment. Little does she know that her best friend is a cursed warlock.
After ten years of him never leaving her side, Sebastian suddenly disappears. Determined to get her best friend back, Isadora uses a tracing spell, which leads her straight into the midst of a secret society calling itself G.A.T.O. For reasons unknown to her, they are holding Sebastian hostage, and to rescue him, she must infiltrate their ranks.