The little boy Tommy and his family always get into some weird troubles. And every time trouble happens, the dog Mr. Prickles, who seems to be very well-behaved beside Tommy, is actually a demon dog possessed by Satan, with various spells and killing skills. He will secretly kill the people who get in the way to save the little master. In addition, the neurotic grandfather knows the true identity of Mr. Prickles early on, but every time he tells the truth, he is regarded as suffering from Alzheimer's disease and fails.