While Puffy makes cake batter in the kitchen, Cheesy is drawn to some cheese on the counter. Puffy slaps his hand away, however, and assertively tells him he can't eat the ingredients before the party. He gets a peanut and cracks the shell, dropping the peanuts in a blender. He turns it on, but it stops because half of a peanut shell is stuck in the blender razor. Puffy reaches for the shell, but, remembering that the blender is still on, he unplugs the cord from the wall.
Seeing a peanut still in the shell, he tosses the peanut into his mouth. All of a sudden, it seems that he is superallergic to peanuts, because Puffy gets a purple rash all over his body, his lips swell up, and his body gets bloated. Cheesy rushes over to help, but, spotting the cheese, he pushes Puffy away and helps himself to a cheesy treat. Later, as he leaves the hospital, Trippy rides up on his unicycle and offers Puffy some peanuts. Wooly, the doctor, quickly slaps Trippy's peanut bag to the ground, making him get mad and run off to find another snack.
Back at the house, Puffy, Fuddles, Superspeed, Cheesy, Lumpy, Trippy, and Scales decorate the place for a party. Lumpy puts up a game of pin the tail on the donkey, not knowing that it's upside down, Scales fills a barrel with water for apple bobbing, Trippy blows up balloons, and Cheesy pours extra cheese in the punch, thinking it's not sweet enough. As they finish their preparations, Puffy sees someone coming towards the house and tells everyone to be quiet. Lumpy turns off the lights shortly before the door opens.
When the lights are turned back on, we see Ale standing in the doorway while everyone shouts "SURPRISE!" Shocked by this, Ale is startled, but luckily she doesn't flip out. Everybody sings "For He's a Jolly Good Fellow" as Puffy brings Ale her birthday cake, causing Ale to shed a tear of happiness. She makes a wish and blows out the candles, while Lumpy begins playing pin the tail on the donkey. Puffy hands Ale a cake knife to cut the cake, leading her to almost flip out. Fortunately, she snaps out of it and begins cutting the cake.
Cheesy, now standing next to a pile of cheese covering the punch bowl, consumes a drop of cheese. He gets an intense sugar rush and begins spinning like a tornado. The tornado he forms sucks some peanuts out of Trippy's hand and causes them to fly into Puffy's mouth as he's about to eat the first slice of cake. He suffers the same allergic reaction he did earlier, popping the balloons with his fur as he bloats up. Mistaking the popping noises for gunshots, Ale flips out into murder mode.
First she attacks Superspeed by cutting part of his face with the cake cutter. She places the slice of face on Fuddles' plate before tying some balloons around her neck. Ale lets go of them and Fuddles flies into a ceiling fan where she is shredded to pieces. Next, as the spinning Cheesy approaches her, Ale holds out the cutter and slices Cheesy like an apple. Ale now turns her attention to Trippy who is reaching for peanuts in the blender, oblivious to all that has gone on around him. Ale plugs the blender into the wall and shoves Trippy into the blender, where he is ground into a bloody mush.
Scales, seeing everything that has happened, hides in the water barrel. Ale searches for more victims to kill when her gaze comes upon Scales' trunk poking out of the barrel. She puts the lid on the barrel, almost drowning Scales who attempts to escape. Ale begins sticking knives, forks, and other sharp objects through the barrel, narrowly missing Scales' body and draining the water. Scales breathes a sigh of relief, but Ale takes the barrel outside and rolls it down a hill. The barrel breaks upon hitting a tree and a dismembered and bloody Scales flies out in numerous directions.
Lumpy, still unaware of what is going on, walks towards the still bloated Puffy. As he pokes Puffy in the eye with the tack on the tail, he explodes, bringing Ale out of her flipped out state. She looks over to see Lumpy impaled on the wall by many of Puffy's fur. Not seeming to notice (or at least care about) the death and destruction around her, Ale uses one of Trippy's wings to mix the fluid in the blender's jar, which comprises of Trippy's blood and the peanuts from earlier. Upon drinking the fluid, she gets the same allergic reaction Puffy did. As the iris closes to end the episode, her lips are caught in the hole and cut off, which then has blood coming out of the lips as they fall.