Hanpeita Tsukuda, a nine-year-old third grader at Tokyo's Oedo Elementary School, begins with a pet bird. His older sister Tsukune accidentally releases it, then finds a strange alien egg and gives it to Hanpeita as a replacement. Hanpeita resents the odd egg, which soon hatches into Ganmo—a pink, chicken-like alien creature who speaks human language. This begins Ganmo's life with the Tsukuda family.

At school, Hanpeita navigates interactions with classmates: American student Linda Skylark, bossy Toshimitsu Saigo, and nerdy Kashio Fujita. Their frequent teasing and bullying add to his daily struggles. Ganmo's arrival further complicates Hanpeita's routine, as the alien attempts household chores and errands but consistently causes confusion and mishaps through his blunders.

The situation intensifies when young Ayumi Ichigaya, sporting purple hair, moves next door. She owns a talking myna bird named Déjà Vu, who openly ridicules Ganmo's appearance. Ayumi's presence and her pet's mockery deepen the chaos in Hanpeita's life, entangling his experiences with Ganmo's ongoing misadventures. These dynamics define Hanpeita's world.