Plankton!

At the Krusty Krab Squidward sends SpongeBob an order for a Krabby Patty, and it ends up in his nose. After SpongeBob makes the patty, it begins to "move" and makes its way towards the front door. Mr. Krabs pulls a tiny person out of the patty, who is none other than his nemesis, Plankton, and uses a plate to fling him back to the Chum Bucket. Mr. Krabs explains that Plankton is his arch-competitor, and has been trying to steal the Krabby Patties secret formula for years.

Later that night, after the Krusty Krab closes, Plankton tries to convince SpongeBob, he and the fry cook are friends, and that as such SpongeBob should come to Plankton's birthday, as it is supposedly the following day. Plankton says that friends give each other things. Plankton asks SpongeBob what he thinks he'd like for a birthday present, and SpongeBob then guesses a booster seat. Plankton is very happy with that proposition, but refuses. Plankton tells SpongeBob that, for his birthday, he wants SpongeBob to give him a Krabby Patty. SpongeBob loudly and boldly refuses and runs back home, but Plankton promises that SpongeBob will "hand deliver it to (him) personally."

Later that night, Plankton enters SpongeBob's head through a pore and makes his way to SpongeBob's brain. He attaches a mind control device to the brain and bends SpongeBob to his will, forcing him to walk to the Krusty Krab, get a Krabby Patty, and bring it to the Chum Bucket's Laboratory. He brings it to an analyzer, which is designed to figure out the ingredients of whatever is dropped into it. As Plankton gloats over his victory and begins to force SpongeBob to drop the Krabby Patty, SpongeBob laments his failure to save it, lovingly describing the Krabby Patty's ingredients and succulence (ironically revealing the Secret Formula). A hungry Plankton leaps out of SpongeBob's head and onto the Krabby Patty, but bounces off of it and falls into the analyzer, thus trapping him inside his computer screen. SpongeBob walks home with the Krabby Patty in hand, ignoring Plankton's angry demands for him to bring it back, his last plea being "I'll settle for some fries!" closing the episode.