After being jailed for solving another mystery, the Mystery Inc. gang stumbles upon another mystery: a gruesome monster lurking underneath the town and cocooning everyone who comes near in green goop.

Strange things continue happening in Crystal Cove, when doorknobs suddenly vanish and a priceless diamond is hidden somewhere. To top it all off, a mysterious Ghost Trucker is also scaring the town.

When an exchange student named Mai Le comes to Crystal Cove, she draws the affection of Shaggy. A dinner party hosted by the Blake Family is interrupted by the Red Wizard warning to stay away from the Dragon's Heart which he has sworn to protect at all cost. Later, Mai Le and the gang to come under attack by a White Wizard who is after Mai Le's ring, and the Red Wizard comes to the gang's rescue, but he is too late to save the ring. The Red Wizard turns out to be Mr. Chen, the owner of Chen's Internet Cafe and Tea House. He reveals that with the White Wizard, now in possession of all four rings, is able to open the Jade Dragon's Heart, which is a priceless ruby. Will the gang solve the mystery or is everything not as it seems?

The gang finally follows the trail to find the hidden treasure, but they didn't think The Freak would go after them. On the other hand, many truths are revealed, including the goal of Professor Pericles and the true identity of Fred's parents. And as if that were not enough, the gang faces a big problem when the parents get angry due to the behavior of their sons. And just when the mystery of the town is at its highest level, does this mean the end of the gang or the beginning of a new problem?

Mystery Inc. has disbanded. Crystal Cove is attacked by a Man-Baby called Crybaby Clown. Mayor Nettles is approached by a mysterious figure who gives her the files on Mystery Inc. while stating that she has to get Mystery Inc. back together.

A polite Englishman in a mask calling himself the Dandy Highwayman kidnaps several women from around Crystal Cove, including Mayor Nettles, Daphne, and Mrs. Blake.