This is a Dizzy Dana episode from Season 1, that premiered on December 28, 1997.
The episode begins at recess, when Mack is shown picking on various background character children-one because he's playing with a pink digital pet toy ("Where'd you get that? Your SISTER?!), another because her shoe is untied, and another because she's dressed formally. Dana, Donnie, and Darrell are huddled up, quivering of fear. They don't see Mack walk into the school to go use the bathroom, but what they do see is a boy who looks just like Mack, except his hoodie is zipped instead of unzipped like Mack's. As the boy walks over to the Dizzy kids, his friend calls out, "Matt! I'll be on the playground if you need me!"
Dana, Donnie, and Darrell mishear what Matt's friend says and are convinced that this nice Matt kid is Mack. Dana asks why Mack is coming towards them, and Donnie is chanting to himself, "Don't pick on me! Don't pick on me!" Matt says that he just wants to know why they're huddling together, scared. Donnie makes up that it's just a game they're playing called "Imaginary Monster", where they pretend that a scary monster is stalking them. Matt joins in on the "game", thinking it is a real game.
As the school bell rings to go back to class, Dana tells her pals that Mack has turned over a new leaf (she still thinks that Matt is Mack). Darrell asks, "What if that kid actually isn't Mack?" Dana says that he definitely is Mack - he's got the trademark spiky brown hair, the gray hoodie, and the black sweatpants.
After school, Dana and Donnie hear a knock on their door. Dana opens the door to see Matt there. The three decide to hang out, but Matt is confused when Dana says she's glad he turned over a new leaf. Dana says it's just an expression, and Matt says "Oh, okay."
The next day at recess, Mack picks on Dana and Donnie when they play pirates. Dana asks, "What happened to the new leaf you turned over?" Mack is confused. Dana tells him to stop playing dumb, and Donnie whispers to Dana that maybe the being nice was just a setup so the kids could be fooled into thinking he is nice as Mack walks away. Then, Matt walks up to them and asks to play Imaginary Monster again. Donnie says he isn't falling for it again, causing Matt to ask "Falling for what?" Dana says "Oh, nothing." and shoos Matt away. Dana and Donnie chat with each other, saying they're sick of Mack "pretending to be nice and playing dumb", still not aware that Mack and Matt are different people.
- The title references the song "Mack the Knife".