The building boom in Amity Park upsets Undergrowth, the ghost of all things plant-like and vegetative. The green-thinking ghost takes over the town, turning everyone into plants themselves. All except Sam, that is, whom he makes his Earth-loving queen. Unfortunately, Undergrowth brainwashes Sam and uses her to attack Danny! Danny, meanwhile, has been battling a nasty case of the chills that leaves him too weak to fight. Escaping from Undergrowth in the Ghost Zone, Danny turns to his cold-loving friend, Frostbite,who explains that Danny is developing a new power. Now, Danny must learn to master his new freezing skill so that he can make his way back to Amity Park and pull the town out of Undergrowth’s leafy clutches.