Hobbes gets to eat with Calvin at the table for dinner. They get to eat early and have grilled cheese sandwiches. Calvin says this is more like it. Mom serves his sandwich, and Calvin notices Mom is dressed nicely. Calvin suspects something.

Calvin is holding onto Mom's leg. Dad is pulling one of Calvin's legs to get him off. Calvin sees Rosalyn. Dad tells Rosalyn the neighbors' numbers are by the phone, and could she pull a leg? Rosalyn asks if she told them her rates have doubled.

Rosalyn offers Calvin a deal. She says if Calvin doesn't give him any trouble tonight, they'll play a game. He asks what game, "Kill the Kid"? She adds that he can stay up half an hour past his bedtime. Calvin obediently sits there.

Calvin gets to choose the game to play. He asks if it's a trick, and Rosalyn tells him they can play his favorite game. He chooses Calvinball. Calvin tells Hobbes to get the time-fracture wickets. Rosalyn asks what the heck is Calvinball.

Calvin tells Hobbes that if he does all his homework, they get to stay up to play Calvinball. Calvin finishes his math homework and starts his history work. At the restaurant, Mom says they're there to have a nice time and not to think of what trouble Calvin's getting into. Dad spins around and says that sounded like another siren.

Calvin finishes his homework, and he says he has to make Rosalyn a mask. When she asks why, Calvin tells her no one is allowed to question the masks.

Calvin sings a Calvinball song about having a game that's never the same, it's always bizarre. It's named after him. He tells Rosalyn to harmonize with Hobbes on the rumma tum tums. Rosalyn thinks this was a mistake.

Calvin has the Calvinball, so everyone has to go in slow motion. When Rosalyn tries to clarify what's happening, Calvin tells her she has to talk in slow motion. In slow motion, Rosalyn says this game makes no sense. It's as if he's making it up as he goes. Calvin says she's stumbled into the perimeter of wisdom. They run. Rosalyn now understands.

Rosalyn says if she's in the perimeter of wisdom, she gets to make a decree. She decrees that Calvin has to catch a water balloon she throws high into the air. As Rosalyn races to get a balloon, Calvin tells Hobbes that she picked up the nuances of this game fast.

Rosalyn tosses the water balloon into the air. As he runs, Calvin says he's in the corollary zone. If he catches the balloon, the thrower has to bend over and hold still. Calvin catches it. He thinks this is going to be sweet. Rosalyn grabs Hobbes and holds him behind her.

Calvin tells Hobbes not to guard Rosalyn. He wants to hit her with the balloon. Calvin says he'll just have to soak both of them. Rosalyn touches Calvin with the flag. She says the baby sitter flag means he has to obey the baby sitter. She says it's a half hour past his bedtime. They have to go in. Calvin grumbles about the darn baby sitter flag.

Dad pulls up and sees the house is still standing. Mom asks how everything is. Rosalyn tells them everything's fine. She says Calvin did his homework, they played a game, and Calvin went to bed. Dad says it's awfully late for jokes. In bed, Calvin says he noticed that when they play games with girls, Hobbes gets captured a lot. Hobbes says some of us are just irresistible.