The Planet of the Sponges is where SpongeBob, Patrick, and Squidward end up on The Planet of the Sponges. Each of them have their own endings to the story.
It starts out with Patrick walking into the Krusty Krab and ordering a Krabby Patty. Squidward rings the bell and gives SpongeBob the order, but SpongeBob sliped on some grease and is unconcious. After a while, Squidward goes in the kitchen to wake SpongeBob up so that he can cook the order, and Patrick wanders in the kitchen, looking for the bathroom. It turns out that SpongeBob was doing grill inspection, and he accidently turned the grill to above max tempurature. It explodes, sending SpongeBob, Patrick, and Squidward into Space.
Meanwhile, Sandy is launching an unmanned (but it has a hollow compartment filled with acorns) space probe into space. In the atmosphere, SpongeBob (who has regained conciousness), Patrick, and Squidward grab onto it and climb inside (releasing the acorns into space). Squidwards nose is crammed, like when the three were in the treehouse.
The probe is caught into a wormhole, rocketing it trillions of lightyears away.
The Planet of the Sponges
The end up on the Planet of the Sponges (which is actually inhabited with Atlantis-like citizens). The inhabitants get them water domes so they can breathe on the barren desert planet. Later, they find out about a prophecy that the planet will be destroyed when three aliens come to the planet. The inhabitants are more then glad to help them get off. They repair the Space Probe, but they must launch in from Mt. Kilueasponga. It is across a wide jungle. Throughtout the episode, they get through the forest, and come to the mountain. The trio narrowly climb it. Time is running out, and when they go to launch the probe, they finds that Patrick let go of it when they were climbing. Even SpongeBob gets angry, but he says that anger doesn't solve anything. The three climb back down, and back out. The mountain is spewing lava, and only Patrick and Squidward can get into it. SpongeBob sacrifices himself to throw the probe into the volcano. He is successful, but the planet is destroyed, sending Patrick to Patrick's Pleasure, Squidward to Squidward's Paradise, and keeping SpongeBob on the now water world SpongeBob's Planet.
In the end, the same wormhole that sent the trio to the planet sucks them back to Bikini Bottom, where they realize that they should not fight and cooperate more.