The Mystery of the Missing Cat — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar, but now they have a home with their grandfather. And now they're looking for a very special cat that has disappeared.

When the children hear that Mr. Woods's cat, Spotzie, is missing, they immediately help out by putting up LOST CAT posters and going door-to-door with a photograph of the cat. But along the way they meet some strange characters, receive frightening phone calls, and even find a cat that looks just like Spotzie -- but is it? It seems the more they search, the more mysterious the mystery becomes!