King Julien finds out about April Fool's Day and makes up for all of the lost years by pranking the zoo animals.
One day in the Lemurs' Habitat, King Julien interrogates Mort on why he ate his fudgy pops. Mort replies "April Fools", but Maurice says it would not work since it is August. King Julien does not know what April Fools is, much to Maurice's shock, so he explains that it is a day in which he pulls pranks on people and that they do not get mad because he says "April Fools". Intrigued, King Julien sets out to pull pranks on the zoo animals, like giving Burt jalapeño peanuts and attempting to trick Maurice into standing on a red X on the ground so that a crate would fall on him (Mort falls for the trap instead).
In the Penguins' Habitat, Skipper reports that a two-ton shipment of Atlantic cod will be approaching the zoo gates in fourteen hundred hours. Kowalski attempts to explain that Operation: Snare the Cod would require steady flippers, laser-like focus, and a sober resolve for the mission to succeed, but King Julien tricks them into falling for a prank three times: he invites Roger for dinner only to make a false alarm, he tricks the penguins into landing on oil slick, and flings fruit at them when they peak. Skipper eventually snaps and subjects the penguins to "Prank Resistance Training". He keeps pulling pranks on the other penguins (like slipping a whoopee cushion on Private's chair, rigging a bucket over a door for Kowalski, and giving Rico a can of rubber worms) until they manage to resist all his pranks. Meanwhile, at Roy's habitat, King Julien draws a rhino's face on Roy's bottom with red permanent marker, infuriating him.
Skipper tests the results of Private, Kowalski and Rico's Prank Resistance Training and tries to trick Private by pointing to his chest where he supposedly has a stain. Meanwhile, King Julien, Maurice, and Mort are walking back to their habitat. Maurice tells King Julien to stop pulling pranks, but King Julien refuses and jumps to the bouncy house. However, he hurts himself because Roy has punched a hole in the bouncy house. He approaches the lemur habitat and threatens to punch the next hole through King Julien.
Back at the Penguin's Habitat, Skipper attempts to get Private to look, but King Julien bursts into the habitat and begs for help from the penguins. However, they refuse to help, suspecting that he is pulling another prank on them, so they kick him out. He also runs to Burt for help, with similar results. King Julien then runs into Roy, and a chase ensues. At one point in the chase, the penguins intervene and fire sleep grenades at Roy.
After rescuing King Julien, the penguins speak with him to make sure he has learned his lesson. However, Roy wakes up and charges at King Julien again. This time, Skipper pushes King Julien out of the way, apparently taking the impact and getting sent flying. King Julien runs over to Skipper, only to realize that Roy has struck a penguin plushie instead. Skipper reveals that it is a prank pulled on him, and all the other zoo animals laugh. King Julien walks away crying, saying he will tell "Christmas Steve" on them. Skipper tells Private it feels good to turn the tables, and Private manages to trick Skipper into looking at his chest for a supposed stain, though Skipper slaps him back anyway.