Mystery, Inc. is investigating a cursed gem and a mummy. So is Dr. Benton Quest. However, it all turns out to be a hoax created by Dr. Zin so he can kidnap Dr. Quest. Jonny wants his father back, so the two mystery-solving teams must team up to save him and stop Dr. Zin's evil plan.
Mystery, Inc. is called to investigate the Keeler Ruby at the Coolsville Museum. The Keeler Ruby was discovered the month before in Egypt by Dr. John Keeler. It is rumored to be cursed. When Mystery, Inc. arrives, a mummy comes to steal the ruby. Mystery, Inc. runs off and has an accident. The person they run into is Dr. Benton Quest who along with his son Jonny, their friend Hadji, and their bodyguard Race Bannon has come to investigate the same thing. The phony mummy kidnaps Dr. Quest and hands him over to his boss Dr. Zin. Dr. Zin then informs the others via a holocommunicator. Jonny locates Dr. Quest being taken the Island of Monsters. Soon all eight heroes are on their way to the island. Oops! I forgot about Bandit. When they reachh the island, they catch Dr. Zin and destroy the lab and rescue Dr. Quest. Dr. Zin is handed over to the authorities, and the ruby is put back in the museum. Dr. Zin vows revenge.