One day in the bustling city of Shibuya, a boy named Neku Sakuraba finds himself holding two black pins that enable him to read the minds of people around him. However, these pins also give him the ability to see the "Noise," hostile creatures capable of "erasing" people from existence. After discovering that he is trapped within the confines of the city, Neku receives a message regarding a mission that will nevertheless erase him should he fail. While trying to fend off a Noise attack, Neku meets a girl named Shiki Misaki who asks him to form a partnership pact with her to fight against the monsters. Shiki explains that she and Neku have become "Players" participating in the "Reaper's Game"—an event spanning over seven days, each featuring a different mission. As Neku and Shiki both strive to survive each day that passes, they begin to meet other Players and Reapers alike, battle stronger Noise, and learn the truth behind the mystery that this game is shrouded in.

Not much is notable about the lecherous Ataru Moroboshi, but his extraordinary bad luck sticks out like the horns in an alien's head. When Earth is threatened by a fleet of alien invaders known as the Oni, Ataru is selected to represent humanity in a duel against one of them. It's a stroke of rare luck for Ataru that the duel is in fact a game of tag, and that his opponent is Lum, daughter of the Oni's leader, who places her personal dignity above victory—as Ataru finds out by seizing Lum's bikini top and with it, victory. However, misfortune kicks in again when Lum mistakes Ataru's promise to marry his girlfriend, Shinobu Miyake, as the desire to wed Lum herself, and decides she rather likes the idea. Wielding her influence as an alien princess, she moves in with him. Forced to deal with the consequences of his womanizing ways, Ataru must balance his crumbling relationship with Shinobu while keeping Lum happy, all the while flirting with every woman he meets.

"I'm coming... Kallen."

After defeating Black Shaft's makeshift Team America, the Golden Japan Jr. team returns to training for the World Tournament that will be held in Tokyo. But during training the Shadow Clan, which uses boxing as an assassination technique, kidnaps Kiku in order to lure Ryuji and the others into a fight to see who's truly the better representatives of Japan's junior boxers.