Nicktoons Unite!, known as SpongeBob SquarePants and Friends: Unite! in Europe, is a 2005 video game featuring characters and levels from SpongeBob SquarePants, Danny Phantom, The Fairly OddParents, and The Adventures of Jimmy Neutron: Boy Genius.
One morning, when SpongeBob SquarePants goes out of his house, he sees that Plankton has taken over Bikini Bottom and used multiple tank-like vacuums to suck away all of the town's civilians. He sees Goddard coming out through a portal and receives a message from Jimmy Neutron. He follows Goddard through the portal to Jimmy's lab and meets Jimmy Neutron, Danny Phantom, and Timmy Turner. Jimmy tells his interdimensional guests about his new invention, The Universe Portal Machine. As he says, it allows people to travel to other worlds. But Prof. Calamitous has been spying on him, stole his idea and created his own Portal Machine. With it, he brought Plankton, Vlad Plasmius, and Denzel Crocker to his dimension. They formed a Syndicate and proceed to steal energy from all the worlds. Since our heroes don't know where Calamitous' lair is, they start with saving all of their worlds first.
Source: Wikipedia, "Nicktoons Unite!," available under the CC-BY-SA License.