Hanpeita Tsukuda, a nine-year-old third-grader at Tokyo's Oedo Elementary School, lives with his family. His older sister, Tsukune Tsukuda, wields a strong, occasionally domineering influence over his daily life. This dynamic manifests clearly when Tsukune accidentally releases his pet bird and gives him a mysterious extraterrestrial egg as replacement.

Initially reluctant about the egg, Hanpeita's attitude shifts dramatically upon its hatching. It reveals Ganmo, a pink, chicken-like alien capable of human speech. Ganmo moves in with the Tsukuda family, attempting to help with chores and errands, though his efforts often spiral into comedic mishaps and misunderstandings. Despite these challenges, Hanpeita gradually forms a bond with the alien.

School presents consistent social difficulties for Hanpeita. He faces targeting from classmates: the dominant Toshimitsu Saigo, the nerdy Kashio Fujita, and American student Linda Skylark. These experiences paint a picture of isolation and vulnerability before Ganmo's arrival.

His social circle later expands with Ayumi Ichigaya, a purple-haired girl who moves next door. Ayumi owns a pet myna bird named Déjà Vu, which frequently mocks Ganmo. Hanpeita's interactions with Ayumi and her pet add further complexity to his daily life. His story, including adaptation into a 1985 animated film, focuses on navigating school life, familial relationships, and his evolving friendship with Ganmo amidst recurring comedic situations.