After the opening ceremony of the Coolsville Museum of Criminology, the gang is startled by the appearance of a masked villain named the Evil Masked Figure. The villain threatens to release monsters that the gang previously captured, causing chaos in the city.
The next day, the gang finds that the costumes of their past villains, including the Pterodactyl Ghost, Black Knight Ghost, Tar Monster, and the 10,000 Volt Ghost, have been stolen from the museum. The villain is believed to be their old enemy, Jonathan Jacobo, who previously claimed to be the mastermind behind all the monsters that the gang had caught in the past.
The gang decides to investigate Jacobo's abandoned mansion, where they find a hidden laboratory. There, they discover that the Evil Masked Figure is actually a robot under Jacobo's control. Jacobo reveals that he did not want to recreate their past villains, but wanted to create new, more terrifying monsters.
The gang must use their skills and teamwork to defeat the monsters and stop Jacobo's evil plan. Along the way, they are helped by a few unexpected allies, including the museum's curator and Velma's love interest, Patrick Wisely.
In the end, the gang is able to defeat Jacobo and his monsters, and the town is saved once again. The movie ends with the gang celebrating their victory and enjoying a well-deserved break from mysteries.