SpongeBob RoundPants was the name of SpongeBob when he got himself new pants in the episode: To SquarePants or Not to SquarePants that were round. He showed them to Mr. Krabs, Patrick, Sandy, Squidward and many other people. SpongeBob then thought that nobody recognised him in the round pants, although only Patrick didn't, and Squidward pretended he didn't recognise him to avoid him and Sandy was just joking.
SpongeBob behaved like he was a new worker at the Krusty Krab and asked for Squidward's help. Following Squidward's advice he started doing things wrong such as being rude to customers, throwing the trash out to the dumpster and sleeping in the bathroom or at the register and even cooking patties like Squidward (burning them). At the end of the episode, Sandy nicknamed him SpongeBob UnderPants.
The narrator says that SpongeBob's pants change happened 3 days ago. SpongeBob is shown wearing square pants on the day this episode starts on. However, the clothing store clerk said they would not get another shipment of square pants for MONTHS!