Mr. Michael Hartford, the famous film director and a good friend of the Three Investigators’, had seek for the boys’ help in helping his friend, Mr Howe, find his missing dog, Red Rover. Mr Howe was also a film-maker who worked with dragons in his movies. One day, while searching for Red Rover, the boys discovered an underground cave nearby Mr Howe’s house which they suspected was used for smuggling goods. The boys decided to investigate the cave and got into trouble when they discovered that one of Mr Howe’s neighbour was involved in robbing gold bars from a bank by using the cave and also a submarine-dragon. Did the boys succeed in solving the mystery of the dragon which Mr Howe had seen the night Red Rover went missing? Did they find Red Rover and return him to Mr Howe?