A race of aliens has come to Earth with the intention of invading the planet, however, they have decided to give its inhabitants a chance, and if they are able to defeat them in a competition, they will be able to save the planet.
Meanwhile, in Japan, we meet Ataru, who is pursuing her eternal love interest Shinobu to apologize to her. He is quite a character, despite claiming her love for her, not a moment passes before he turns his gaze to see another girl.
Shinobu is fed up with Ataru, and although she gives him one last chance, Ataru returns to his old ways without thinking, she reacts and punches him before running off. As Ataru wonders why Shinobu is so angry with him, the monk Sakuranbo appears and throws him into the river.
Sakuranbo, or Cherry, begins to talk to Ataru, and suddenly tells him that the features of his face only attract bad luck, and although he denies it, bad news come his way rapidly.
Ataru’s mother appears, accompanied by Shinobu, who asks him if he heard the news yet, but before he can ask what they mean, government agents come to him and take him back to his place.
There, Ataru sees a huge man sitting, and looking closely, he has horns and looks like a demon, or oni, traditional in Japanese culture.
As it turns out, he is leading the alien invasion, and randomly, they have selected Ataru to face them in the competition to save the planet.
According to the rules, Ataru will have ten days to catch and grab the horns of Lum, the alien’s daughter, and if he does, they will leave Earth alone.
Seeing how attractive Lum is, Ataru begins to think about how much fun trying to catch her can be, and he doesn’t hesitate to say that he will take her tightly in his arms, and of course, this immediately annoys Shinobu.
As the days go by, Ataru’s parents do not understand why their son has such bad luck, and when they talk to Cherry they think that he was born under the star of bad luck, and his mother confirms that he was born on Friday the 13th, the day with the least amount of luck in the Buddhist calendar, then they recall that a black cat passed by them and even an earthquake occurred before he was born.
Ataru complains knowing that if he fails, he will be the most hated man on the planet, and Shinobu tells him that it is his fault for not thinking twice before accepting the challenge, and above all, for ogling Lum.
But despite this, she promises Ataru that he will not be left alone, and that if he wins, she will marry him one day. That motivates Ataru to give his best.
The next day, Ataru is determined and keeps repeating the word “marriage” as he chases after Lum. At last, Ataru is able to grab the girl by one leg, but before he can do anything else he is shocked by her power and ends up on the ground.
Despite everything, Ataru gets up and continues to claim that he will get married, in an oversight, Lum loses part of her outfit and when she tries to get it back ends up losing the competition.
As Ataru celebrates in front of the audience, he says that at last he will be a married man. And after seeing him so determined, Lum tells him that she accepts his marriage proposal.
And although Lum’s father is surprised, he accepts that they have lost, and that the winner has the right to take everything, and asks him to take good care of his daughter.
Ataru claims that it is all a misunderstanding, and now he will have to deal with an alien fiancée, who can be extremely jealous.