Ataru Moroboshi's mother looks back on how her formerly peaceful middle-class life ended half a year earlier, when alien invaders called the Oni arrived on Earth. Their leader, Mr. Invader, proposed that if a randomly chosen human could defeat his daughter Lum in a game of tag, the planet would be spared. As luck would have it, the fate of the world fell to the housewife's troublesome son. Surprisingly, young Ataru won the challenge—and as a result, Lum became an unexpected addition to the Moroboshi household.
Ataru's mother now reflects on all the characters her son has gotten involved with—allies and enemies, human and alien—and the stress their antics have caused her. Unsure whether to admire her son's resilience or consider him pitiful, Mother Moroboshi can only wonder if her family will ever know peace and quiet again.
Kaede is a young kunoichi who doubts her chosen path. Her elderly, ill-tempered master Yatsude insists she remain a ninja, and to make matters worse, Kaede has further angered her master by losing a confidential scroll in the woods.
Meanwhile, Ataru finds his school excursion to the ancient capital of Nara tedious. But stumbling upon the scroll and catching Kaede's attention makes his field trip a bit more interesting.