Mr. Cat seems to be growing up into a teenager, but teenagers are not allowed in Smileyland. The others desperately try to find a way to let him stay.
"Let's Play Goodbye, Mr. Cat!" contains the following tropes:
- Bullying a Dragon: After turning invisible, Mr. Cat actually took the time to taunt and even hit Bad Kaeloo. Once Kaeloo manages to figure out that the invisibility is temporary, she willingly transforms back into Bad Kaeloo and promptly beats up Mr. Cat once he is visible again.
- Exact Words: Kaeloo says that Mr. Cat will disappear. Near the end, he does - he's completely invisible, but alive and kicking.
- Gross-Up Close-Up: Kaeloo informs Mr. Cat that he has a zit on his face. The audience is then treated to a close-up of the zit on Mr. Cat's face.
- Invisible Jerkass: Once Mr. Cat turns invisible, he goes around attacking Quack Quack and kicking Kaeloo.
- Laser-Guided Karma: Bad Kaeloo repeatedly smashes Mr. Cat into the ground and chokes him with a jump rope for being an Ungrateful Bastard and outright attacking the same people who have been trying to help him.
- Mood Whiplash: The episode starts with a seemingly cheerful plot about minigolf. Then, the title card pops up, revealing that the episode is called Goodbye, Mr. Cat! The title card narrator is sobbing as he says it.
- Puberty Superpower: Mr. Cat and Stumpy get invisibility powers after hitting puberty. Subverted though as it turns out that they haven't actually hit puberty, they've just temporarily contracted a disease with symptoms similar to puberty, such as acne. Side effects of said illness include invisibility.
- Still Sucks Thumb: Mr. Cat sucks his thumb to prove to the others that he is really a child.
- There Are No Adults: Adults (and teenagers, for that matter) are not allowed in Smileyland.
- Ungrateful Bastard: Kaeloo and Quack Quack help Mr. Cat out, and he repays them by beating them both up for no apparent reason.
- Wham Episode: The viewers find out that Mr. Cat is less than 13 years old.
