Responsible Taihei has to deal with his lazy little sister, Umaru, who trashes his apartment and throws tantrums to get manga and junk food.

Taihei comes home to a complete stranger: Umaru’s public face. She’s cleaned the apartment and hidden her games to be able to invite her classmate and neighbor, Ebina, over.

Umaru’s sulky, glaring classmate, Motoba, has unexpectedly shown up at her door… while she’s wearing her hood! Is her secret out?

Umaru, aka UMR, is a force to be reckoned with in the arcade. She’s invited to participate in a fighting game championship only to bump into her school rival, Tachibana.

Summer vacation means hours at the arcade! Umaru instructs in the fine art of enjoying summer like a (lazy) pro. Later, Taihei endures another typical day at work.

Taihei and Umaru experiment with free time and personal space in their little apartment, and playing “The Game of Life” hits too close to home for them all.

People in his office have seen Taihei walking home and going shopping with a hot girl! His coworker comes over for a visit to see this so-called “sister” for himself.

In the Doma household, winter means lazy days beneath the kotatsu! Umaru reflects on her interesting year while Taihei tries to collect on a promise she made to him.

Umaru gets sick, but it’s a good thing her germs are as lazy as she is! Sadly, her germs show her brother no mercy. Later, Taihei remembers a day at the arcade, 10 years ago.

Umaru and Ebina discover that their appetites match, the council in Umaru’s head furiously debates about the perfect snack, and Taihei tries to get Kirie excited for summer.

In this last episode of the season, Umaru might have to reveal her secret when her separate circles of friends come together during a much anticipated trip to the beach!