Ataru Moroboshi's mother reflects on how her once calm middle-class life was disrupted half a year ago, when the alien Oni invaded Earth. The Oni leader, Mr. Invader, proclaimed that if a randomly chosen human could defeat his daughter Lum in a game of tag, Earth would be spared. To everyone's astonishment, that crucial role fell to Ataru, who actually won, and as a result, Lum became part of the Moroboshi family.
Now, Ataru's mother thinks about the various individuals—allies and enemies, human and alien—that her son has gotten involved with, resulting in endless turmoil at home. She wonders whether she should admire his resilience or pity him, and whether her household will ever know peace again.
Elsewhere, Kaede, a young kunoichi, questions her fate as a ninja, especially after her irritable elderly master, Yatsude, insists she continue on that path. Things get worse when Kaede accidentally loses a secret scroll in the forest, further infuriating her master.
During a school trip to the historic city of Nara, Ataru initially finds the excursion dull until he stumbles upon the missing scroll and catches Kaede's attention, adding an unforeseen twist to his day.