Ataru is a man of simple pleasures. When it’s sukiyaki night at the Moroboshi house, he’s super-pumped. Unfortunately he’s never able to partake in the feast, as Lum grabs him and leaps through an inter-dimensional portal she made in his closet that leads to the Oni homeworld.
Lum is in battle gear, and soon so is Ataru. The lines are drawn between the Oni and the “Lucky Gods”. Ataru feels like some kind of bloody, horrific war is going to start, but the “battle” takes the form of…tower basket ball toss, an even not out of place on school sports days.
This is kinda boring to Ataru, until he spots a major babe in Benten, one of Lum’s old friends (Ishigami Shizuka). Back home, Ataru’s parents wait as long as they can, then eat all the sukiyaki, before hearing their son’s voice and freaking out. Cherry arrives to help them speak to their son, now allegedly dearly departed to the hereafter.
In reality, they can just hear him through the portal as he flirts with Benten. While she’s understandably “who is this guy” at first, once she realizes he’s Lum’s husband she decides to have a little fun at her expense and plays along. This results in Lum and Benten, the two basket minders, ignoring the game completely to fight over Ataru.
Before Cherry summons Ataru’s voice again, he has Ataru’s folks make more sukiyaki, at which point his mom has lost her patience and holds the tiny priest at knifepoint. He does the same nonsensical chanting as his niece Sakura, tuning into Ataru just as he’s facing his “punishment” as the weakest link on the losing team: being pelted with pellets by both sides.
As is typical of Urusei Yatsura, the next morning is a bit of a reset, but Ataru is in bed with a cold. Somewhat surprisingly, Shinobu is by his side tending to him, and Lum is nowhere to be found. Soon Mendou, Ataru’s friends, and Cherry are crowding the room, just as it starts to grow very cold and snowy.
Lum went to Neptune to visit a friend through the portal, so a bit of the icy world seems to be “leaking” into his room, including an avalanche’s worth of snow that buries Ataru. He’s dug up not by Shinobu or his friends, but by a new character who resembles a yuki-onna. She goes back through the portal and then down a deep chasm.
Starting with Ataru (who is pushed), everyone follows suit, and lands upside-down on the snow-packed surface of Neptune. There, Ataru reunites with Lum (in a smart tiger-print two-piece combo more appropriate for the climate than her usual bikini), who reveals the yuki-onna is her old friend Oyuki (Hayami Saori, of course).
Neptune is a world full of nothing but women, which makes it a paradise for Mendou, who is all to happy to dig snow for them endlessly. Meanwhile, Oyuki invites Ataru, Lum, Shinobu and Cherry into her futuristic mansion. Ataru can’t help but flirt with Oyuki, incurring the rage of both Shinobu and Lum (as well as Lum’s lightning).
Ataru begs to go somewhere where he’ll be safe from their wrath, praising Oyuki for being a pure, gentle, and above all non-violent maiden. However he soon finds that Oyuki, who ditched her outdoor robes for a revealing ice-blue one-piece, was planning to seduce Ataru all along. Things are about to get racy when the wall crumbles before them and B-Bo, Oyuki’s yeti attendant, takes exception to Ataru’s presence.
B-Bo chases Ataru through the Neptunian wastes and back through the portal to Earth, where news choppers capture the ensuing rooftop spectacle. Once the King Kong style incident is over, Ataru finds himself in a full body cast, tended to by both Shinobu and Lum, who hoped he learned his lesson about chasing every girl with a pulse. Of course, he didn’t learn, and will never learn—otherwise he wouldn’t be Moroboshi Ataru!
The third and final segment is the shortest, and takes place after the credits. At the end of the semester, Ataru has an announcement for everyone: he’s retiring. His teacher thinks this means he’s dropping out due to his upsettingly terrible grades, but it’s Mendou who shatters the fourth wall by assuming Ataru was retiring … as the main character of Urusei Yatsura.
Everyone goes along with this, because everyone wants to be his replacement. It results in a callback to every character large and small we’ve met so far in the first six episodes, each making their case. Finally Ataru has to disappoint them all: he’s not retiring from being the MC, but from the school presidency.