One day in Bikini Bottom, SpongeBob wipes the grill with a sponge (made from part of his own tongue) at the Krusty Krab. He proclaims that it is freshly clean and tries to convince a customer to eat a Krabby Patty that was on the grill. When SpongeBob went back inside to cook that patty, the customer asks Squidward why he doesn't want a Krabby Patty, and he replies that he has seen what SpongeBob cleans the grill with. It goes back to SpongeBob, who is on his way to the freezer for a patty when he slips on a puddle of water. Upon closer inspection, SpongeBob observes that the flood is coming from the sink, which is clogged with water. As he is trying to figure out how to unclog the drain, Patrick comes in and slips, but because he was his normal clumsy self.
Eventually, Mr. Krabs overhears them trying to pull the drain, and stops them before they have a chance. They hear the story of the Main Drain, and Mr. Krabs tells the story. The story was that there was a plug that two kids supposedly pulled long ago that ended the world of Bikini Bottom. When they ask him where it is, he becomes nervous and tells them to go away. The two go outside and SpongeBob steps on Plankton, and then they ask him about it. He becomes nervous also and tells them to go away. SpongeBob says that he heard it from Mr. Krabs, and Plankton claims Mr. Krabs was lying. They ask him where it is and again, no results. They turn to Old Man Jenkins, but he yields no results.
Then they go on the adventure for the plug by dragging Patrick's house. Just before they pull the plug, Mr. Krabs and Plankton stop them and tell the story of how they were the ones that pulled the plug when they were younger. They explain that they lied because they didn't want SpongeBob and Patrick to look for it. Unfortunately, Patrick already pulled the plug and everything (including themselves and the Bikini Bottomites) was sucked into the drain.
Patrick then sits up in his bed, after hearing SpongeBob's story of the drain, and claiming it's the most realistic story he has ever heard, indicating everything happens on the episode was only SpongeBob's story.
Running gag: Patrick turning on a music from his radio when Mr. Krabs is about to tell the story of the Main Drain.
Though it's revealed that this episode was only a story being read to Patrick by SpongeBob, the entire population of Bikini Bottom was sucked away before: in the episode "A Flea in Her Dome," everyone was sucked into Sandy's Treedome. This event could have been the inspiration for this story.
The Main Drain, which sucks in all of Bikini Bottom, obviously refers to the end-of-the-world scenario "killer strangelet."
Like his arms and legs, SpongeBob's tongue can also be detached and reattached.
It's odd with Patrick's antenna, in some episodes, it breaks quite easily, such as "Christmas Who?" with a bauble, but it can hold bubbles and a heavy rock with Patrick and SpongeBob on it.
This is the second time Mr. Krabs and Plankton were shown as friends and kids. The first was "Friend or Foe."
This is one of the episodes where Mr. Krabs warns SpongeBob and Patrick about doing something, but they don't listen to Mr. Krabs. The first being "Hooky," second being "Sailor Mouth," and the third being "Face Freeze!"
After a flashback shows Mr. Krabs and Plankton pulling the plug, it is unknown how they got back.
It was revealed in this episode when SpongeBob and Patrick are seen flying on Patrick's house intended with bubble made balloons. This scene of Patrick's house is very similar to Mr. Fredrickson's house from a Disney/Pixar film "Up (2009 film)."
When SpongeBob said, "Patrick we need to find that drain," the road suddenly disappears.
In the scene where Patrick's rock has balloons with ropes, they are attached on the antenna on Patrick's rock. But when Patrick is acting like a horse, the antenna disappears and SpongeBob is suddenly the one who is holding it.
When SpongeBob slipped on the water, the freezer was closed. SpongeBob actually left it open. But it is unknown how did the freezer close itself.
When SpongeBob put the danger sign on the floor, the lever beside the freezer disappears.
When he is about to check the clogged sink, SpongeBob still needs to hold his breath even though they're already underwater.
When Mr. Krabs turned off the lights, it is the left side, but when he finished the story, the switch suddenly changed positions. The next scene shows that the switch is above SpongeBob, hanging on the ceiling.
SpongeBob and Patrick are looking for the Main Drain, but the customer is still waiting for his Krabby Patty. This happens in lots of episodes, when SpongeBob gets distracted while a customer is waiting for his food and then it's like the customer didn't even exist.