The Mystery of the Poster is the first episode in Season 5, which is premiered on 12 August 2016.
Don discovers his expensive Next HarGreeny poster smeared with ketchup, he pulls out his officer's cap and enlists Pen to help his investigation find the suspect.
The episode starts outside Dee's cabin as Lisa, walking while playing The Princess Rescue game on her iPhone and suddenly was shocked when seeing Don's Next HarGreeny poster, covered with ketchup, and then fainted, a few hours later, other villains, seen laughing at Don's poster and then came Don himself, figuring out what they're laughing at, come to figure out that it was his expensive poster that was vandelized, and leaving him disgrunted, and wiling to find the main culprit of this, so allies Pen to help him solve the case.
- In the second scene, we see that Lisa and Pen draw a heart between the photos of Flick and Dee.
- This episode is very similar to the episode of The Interrogation in the show Clarence.
- In the first scene, Don and the others need to figured out who was the one who covered Don's Next HarGreeny poster in red ketchup.