In an island world called Volcano Island, a trio of crabs, led by the Wise Old Crab and his two assistants are gathering around a ritual at Summoner's Rock to summon 'The Nine' heroes to defeat their ancient enemy, the Mawgu. However, the Mawgu breaks the circle of summoning, scattering the nine heroes across the island. SpongeBob SquarePants and Danny Phantom end up being summoned in front of The Wise Old Crab. He tells them that he has summoned them here and tells them that the Mawgu has escaped from his prison and is seeking vengeance against the island. The Mawgu has created a terrible ooze corrupting everything and everyone that it touches. However, according to the crabs' legends, the "Chosen Ones" can rescue everyone from the Mawgu Ooze and defeat the Mawgu.
SpongeBob and Danny fight the Mawgu's minions, finding Timmy Turner, Sam Manson, Tucker Foley, Sandy Cheeks, Squidward Tentacles, and Patrick Star. They find one of Jimmy Neutron's communication devices, learning that the Mawgu's interruption of the ceremony left Jimmy back in his lab. Jimmy also reveals that the Mawgu had created a rift in time and space above the volcano which is siphoning energy from Retroville, Bikini Bottom, Dimmsdale, and Amity Park, to power himself; if this continues for too long, it will suck up their worlds and destroy the multiverse.
Using Jimmy's blueprints, Tucker sends the rest of the heroes to collect parts for a Rift Zipper, to seal the rift and stop the Mawgu. Once completed, the eight heroes enter the volcano and travel up into the rift, where the Mawgu awaits them. Sam, Tucker, Sandy, Timmy and Patrick keep the Mawgu busy while Danny, SpongeBob and Squidward ascend using the debris sucked in from the other worlds to the rift's core. The device drains the Mawgu of his strength, allowing the closing rift to seal him away forever. After being congratulated by the Wise Old Crab and the denizens of Volcano Island, the heroes then depart through a portal opened by Jimmy, sending them back to their respective worlds.
