Gus and Mouse have moved into the Historical Museum, living there with Mr. Frizzle and Cora the cat. Late one night, Cora came in from a moonlit walk and said, “A baby ghost’s outside.”

“What do I do with it?” Gus cried. With Cora’s help and a book about caring for ghostly babies, Gus manages just fine. However, cranky Mr. Frizzle soon demands that Gus gets rid of baby ghost. How do the two caretakers of the Historical Museum settle their differences? Well it turns out that not only is Gus a friendly ghost, but a smart one too!