"Oobi Tries Lean" is the fourth episode.
Oobi does the lean.
Oobi is hosting a wild dance party set to trap music. Three other hands are there with him, making a ruckus. Grampu is trying to sleep in his bedroom when he hears them and goes downstairs. He tells Oobi's guests to be quiet and marches back upstairs, where he fluffs his pillow and goes to sleep after saying "God, I miss Brendan Fraser."
Oobi's party guests are mixing "lean" (which is obviously just a blue sports drink) in a glass. Oobi asks them what they're doing and they claim they're making purple drank. Oobi says it sounds great, so he steals their glass and pours it all over himself. Meanwhile, a masked hand puppet is stealing all of the candles from Grampu's dinner table without Oobi knowing.
Oobi starts to experience the psychedelic effects of his drink. His entire world becomes purple and swirly as the Pure Imagination song by Willy Wonka plays. He falls asleep on the couch and starts snoring/roaring loudly. 15 hours later, he wakes up and sees a glass of milk. He says nothing but "Why is the milk... why is the milk..." and falls asleep for another 72 hours.
Uma walks downstairs and sees Oobi. She asks him about the party he had and he responds by mumbling. "Wow, looks like somebody had a rough night," she says. Oobi goes back to the couch and has a lean-induced vision: the ghost of the original Grampu from the first two episodes appears and warns him that the new mail-order Grampu is evil. "He will use that which is most precious to you to destroy the earth," says the ghost.
Still mumbling and gurgling from the lean, Oobi tries to tell Uma about his vision. "We gotta find the source of the mail-order Grampu power," he says. While Uma tries to understand what he means, she notices that all of the table candles are gone. Oobi decides that they must be the mail-order Grampu's power source. "We have to stop this," says Uma.
"You're too late to stop me now," says the mail-order Grampu from the other room. He removes a mask and reveals that he's actually the rapper Desiigner in disguise, and he plans to destroy them. "Desiigner, I knew it! You made me take lean so I wouldn't know about your plan!" says Oobi. Harnessing his candle power, Desiigner starts pelting candles at Oobi and Uma.
Uma says to Oobi that they just need to move around quickly so Desiigner, with blurry vision caused by lean, can't see them. They move around and Desiigner uses up his candle supply, missing them with every shot. "That's it. You have defeated me. Lean is wack my dudes," says Desiigner. He blows up.
"We did it! High five!" Uma says to Oobi. She waits for Oobi to high-five her, but he is on the ground, finally getting a good night's sleep after the lean wore off. The episode ends with a public service announcement about getting Brendan Fraser to return to acting.