Mallory and the Ghost Cat is the third book in the Baby-sitters Club Mystery series.
Mallory babysits for the Craine family. The three little girls: 6-year-old Margaret, 4-year-old Sophie, and 2-year-old Katie are a lot of fun and the parents are nice. The girls' regular sister, Aunt Bud is unable to sit due to a broken leg.
One day, when Mallory is sitting for the Craine girls, she hears what she thinks might be a cat, but she can't find one anywhere in the house. Margaret and Mrs. Craine claim that they don't own a cat. On Mal's next sitting job, the girls hear the cat noises as well. They search the attic and find a white cat. The girls put the cat in the laundry room with food and decide to call him "Ghost Cat."
Mr. and Mrs. Craine allow the girls to keep the cat and they're excited to show him off to Mallory when she comes to babysit again, but the cat ends up escaping from the laundry room and hides whenever the door is open. Mallory and the girls go on another cat hunt and end up back into the attic.
Even though they don't find Ghost Cat, they find a bunch letters written by a former owner of the Craines' house named Kennedy Graham, whose only companion was his cat, Tinker, which makes the girls wonder if Ghost Cat is really a ghost. Mallory invites Dawn over to run tests on the cat and even though they prove the cat is real, Mallory still hears noises from the attic even though Ghost Cat is sitting right in front of her.
During Dawn's visit, Mallory gets a call from a man who claims to be Ghost Cat's owner. The man describes Ghost Cat (whose real name is Rasputin) and he arranges to come get him in 2 days. Mallory is there when the man picks up the cat and the man looks exactly like the picture of Kennedy Graham that was found with the letters which makes Mallory wonder if Ghost Cat was really a ghost after all.
Mallory's Uncle Joe comes to visit the Pikes. He acts cranky, does & says weird things, and doesn't interact with the rest of the family. Near the end of his visit, Mr. and Mrs. Pike tells the kids that Uncle Joe is in the early stages of Alzheimer's Disease and moves in to Stoneybrook Manor where he can get the help that he needs.
One night when Mallory is baby-sitting for the Craines, she hears a cat crying somewhere in the house. But the Craines don't own a pet. So Mallory and the girls go exploring — and discover a mysterious white cat hiding in the attic. They name him Ghost Cat, and the mystery is solved. They think.
Until Mallory and the girls continue to hear eerie cries coming from the attic. If Ghost Cat is sitting right there with them, who — or what — is upstairs in the attic?