Nimbus, a young black cat with a crooked foreleg and a coat that always looks scruffy, no matter how much she grooms it, loves staying inside her home, safe from the mean and dangerous world outside, content to spend her days alongside her one true friend, her human boy, Fletcher. But one night, when a goblin-like mara attacks Fletcher, Nim leaps to his defense, battling the nightmare creature. The mara quickly fades to smoke and drifts away, leaving Nim badly injured but relieved she's destroyed the monster and protected her boy. But rather than seeing Nim as a hero, Fletcher's stepmother believes it was Nim who attacked Fletcher. While he sleeps, the angry woman drives Nim to the countryside and abandons her in a rat-infested dump. Too weak to make the long journey home, Nim finds her way to the rundown cottage of a frightening old woman and her four eccentric housecats. She soon discovers that when she blinded the mara's eye, she inadvertently stole its dream sight, giving her the ability to walk in the dream realm. Realizing she didn't defeat the mara and that it's returning every night to feed on Fletcher's lifeforce, Nim is determined to get home as quickly as possible.

To do that, she'll have to rely on the Diversions of the Cat, four useful skills taught to her by the old woman's housecats. Then, using her dream-walking abilities, this time she will destroy the nightmare creature once and for all, before it can destroy Fletcher.