When aliens from the planet Oni come to invade Earth, Ataru Moroboshi is chosen to save his planet if he wins a game of tag against their princess, Lum. After succeeding, due to a misunderstanding, Lum thinks that Ataru has proposed to her, much to his girlfriend Shinobu Miyake's irritation.

Lum insists on spending time with Ataru in school, angering Shinobu and making his life miserable. While trying to go into hiding, he meets a shrine maiden named Sakura who insists on performing an exorcism on him.

New transfer student, Shutarō Mendō, is instantly popular with the class' girls but disliked by the boys who set Ataru as his opponent for class president. Since the class is equally divided between male and female students, the election ends in a draw, and Shutarō decides to settle the result with a duel.

The school hires Sakura as the new nurse, causing a commotion among male students. The PE teachers ask her to exorcise a pair of boxing gloves, but Cherry and Kotetsu-Neko make one of the gloves go flying and hit Lum. Later, Ataru ends up wearing both gloves which results in much grief for him, but also Lum's delight.

Lum brings Ataru to her home planet through a portal to battle with the Fukujin alien clan. There he flirts with Lum's childhood friend Benten of the Fukujin and faces punishment for making the Oni lose. Believing that Ataru is dead, his parents and Cherry perform a ritual to guide his soul to the afterlife.

Bedridden with a cold, Ataru is visited by his classmates Shinobu, Shutarō, Satoshi (Megane/Eyeglasses), Kōsuke (Perm), and Akira (Chibi). After finding an interdimensional portal Lum left open in Ataru's room closet, they are brought to Neptune by another of Lum's childhood friends named Oyuki.

While hanging out with Lum, Shinobu, Shutarō, Cherry, and Sakura at one of the Mendō family's hotels, Ataru finds a water-dwelling yōkai secretly living at the bottom of the pool. After Shutarō kicks it out, the little yōkai finds a new home.

Lum enrolls as a student in Ataru's class, nearly causing him to choke to death. During the school's sports festival, Ataru hits on Ran, a childhood friend of Lum, who transferred to the class next door to take revenge over a boy they both liked in the past.

Lum's ex-fiancé, Rei, comes to Earth to propose to her. Although he transforms into a huge beast when upset, he is incredibly handsome in his normal state. However, Lum categorically turns him down because she can't stand his gluttony. After a failed attempt to win Lum over and burning with jealousy, Rei chases Ataru and Lum around the local park, causing much mayhem among couples and food-vendors.

As Ataru and Shinobu's mothers arrive to school for parents day, Shutarō's mother's ox-cart gets crushed by Lum's mother's spaceship. Lum is asked to translate the Mendo matriarch's grievances, since her mother doesn't speak Japanese, but her poor translation skill causes many misunderstandings.

Shutarō's mischievous younger sister, Ryoko, visits him at school and catches Ataru's interest, causing much mayhem.

Irritated that his parents are overjoyed at having Lum living with them, Ataru storms out of the house and finds her little cousin Ten in the mailbox. Later, they go to Sakura's temple for the New Year, where they meet with Cherry, Shinobu, and Ran, who is working there part-time and does some quick thinking to keep secret that she is an alien. Ataru snaps after Ten foils his attempts at flirting with the girls, and Sakura gets a new admirer.

The school decides to crakdown on snacking during lunch time, and post teachers at the students' favorite spots to catch them. Ataru leads a resistance which escalates in an all-out war in the shopping district.

During a baseball game at the Tomobiki High School's sports tournament, Tobimaro Mizunokoji shows up to challenge Shutarō because of a childhood incident. Tobimaro's skills and meddling from Shutarō's sister, Ryoko, make for a memorable game...

After Rei eats a crumb of taiyaki off Ran's cheek, she mistakes it for a kiss and assumes he is over Lum.

At a café with Lum, Ran revisits their childhood and how she constantly got in trouble because of Lum's doing. Shaken by how she wronged her friend, Lum forgets to pay the bill on her way out.

On one of the last days of Summer, Lum, Shinobu, Ataru, and Shutarō meet the Fujinami family, a father and a teenager, running the only open seaside café on the beach. The teen, named Ryunosuke, doesn't want to take over the family business which causes conflict with the father. Ataru and Shutarō decide to leave until they find out Ryunosuke is a girl, but their attempts at flirting are unsuccessful. The group then try to get parent and child to solve their issues peacefully, only to cause more mayhem.

Ryunosuke decides to buy her first bra. She meets Shinobu who is being stalked by delinquents from Butsumetsu High. After learning that the delinquents' bossman is hopelessly in love with her, Shinobu goes on a date with Ryunosuke in the hopes of making the unwanted admirer give up on her, with Ryunosuke wanting to use the money given to her for the job to buy a bra. Ataru, as usual, can't resist interfering.

Lum creates a lipstick that makes people wearing it attracted to each other like magnets. The next day at school, Ataru tries it on Ryunosuke but the whole class ends up wearing it.

Shutarō invites his classmates to meet the Mendō family's prized octopuses. With Ataru in the lead, they insult the most precious octopus on the estate, Matsuchiyo, who runs away with his feelings sorely hurt. They all go to find Matsuchiyo after Shutarō promises a reward for the little octopus' safe return.

After learning of a mining operation on a planet with a secret from when they were in elementary school, Oyuki meets with Lum, Benten, and Ran about their revenge attempts on their old robotic teacher, CAO-2.

Ten accidentally shrinks Lum with a jar that makes things smaller. While he rushes to get another jar to turn her back to normal, she tells Ataru that the only way for her to regain her regular size is for him to shower her with love and affection.

Ataru steals Ten's gum that can create whatever the user imagines when chewing and blowing a bubble.