Pranksters is episode 41a of Rocko's Modern Life.
Heffer suspects that he is being targeted for revenge after his practical jokes get out of hand.
On April 1, Filburt is unhappily working at Stuff on a Stick at the mall's food court. When Rocko and Heffer arrive to get their meal, Heffer plays a prank on Filburt before they laugh, saying, "April Fool's" to him. Filburt, after shouting to his friends, mopes about being pranked every year. At the table, Heffer plays a joke on Rocko, making him think Melba Toast has arrived at the mall, which she hasn't. Then, Heffer gets pranked by Rocko when he finds and screams about a rubber rat on his stick.
Later, when Rocko and Heffer arrive at Kind of a Lot O' Comics, Rocko's boss Mr. Smitty gives Rocko an old letter which describes about his grandmother about to arrive from the airport. After trying to ask for his boss for a day off (while he thinks it's a joke) and calling for Filburt, Mr. Bighead, the Chameleon Bros, and the head elf (from the Christmas episode) with them thinking it's a prank, he reluctantly asks Heffer to pick her up while telling him to not pull pranks.
At the airport, Heffer meets with Granny Rocko, whom gives him a hard time (while mistaking him for Rocko), forcing him to carry a huge load of her luggage up the regular stairs instead of an escalator and later being forced to let the granny drive as they nearly get into some accidents. Later, they arrive to Rocko's, whom Heffer, exhausted, lies on the chair after placing her bags down. He then looks at Rocko's photo, thinking that Granny Rocko is actually Rocko in disguise, and plans to pull an ultimate prank on Granny Rocko.
While Granny Rocko bakes some muffins, Heffer uses some long rope around the house along with a rocket, hoping to lure the granny to the living room. Just as he lights the fuse, it turns out Rocko and Granny Rocko (whom mistakes Rocko for Heffer) are two different people. In tears, Heffer takes the full prank, which was originally gonna be used on Granny Rocko. Then, the rocket takes off with Heffer being pulled all over the house, then to space where he crashes to the sun, then heading back to Earth, crashing through China, going through the crater all the way back to Rocko's front yard.
As Rocko, whom watches with Granny Rocko, is shocked and tries to get him off the rocket, Granny Rocko suddenly laughs hysterically and in a disturbing fashion before her skin, which turns out to be a costume, is pulled off, revealing that Granny Rocko is actually Filburt, who uses the same snout flipping trick on Heffer. The maniacal excitement overwhelms Filburt's system and he falls down, nearly passing out and after panning away from O-Town, the Earth, and through space with a blank screen (after a white flash) was seen, Heffer is heard saying (and claiming), "I knew it was him the whole time".