Although Fritz is abused by his Master, Will is treated as a privileged pet by his. Eventually, Will's Master reveals a plan to replace the Earth's atmosphere with the Masters' toxic air to enable full control of the Earth. When the Master finds Will's diary, Will kills him to maintain the secret. With the assistance of Beanpole and Fritz, who temporarily stays behind to maintain Will’s alibi, he escapes and returns to the White Mountains with Beanpole. The story ends with Will and Beanpole giving Fritz up for dead and returning to the White Mountains, although he returns at the beginning of the next book.