Takopi, an alien from the Happy Planet, crash-lands on Earth in 2016 with the mission to spread joy. He is found by Shizuka Kuze, a lonely fourth-grade girl who is bullied at school and neglected at home by her absent mother. She names him Takopi and finds her only comfort in her dog, Chappy. Determined to make her smile, Takopi uses his Happy Gadgets – advanced tools from his home world – to try to solve her problems. But his naive understanding of human suffering leads to disaster: Shizuka, overwhelmed by pain, uses one of the gadgets to kill herself.

Desperate, Takopi uses a Happy Camera to rewind time and try again. Over repeated loops, he encounters Shizuka’s classmate and chief tormentor, Marina Kirarazaka, whose own home life is abusive. During one confrontation, Takopi accidentally kills Marina with the camera, breaking it and making further time travel impossible. To hide the crime, Takopi enlists the help of another classmate, Naoki Azuma, a quiet boy who idolizes his older brother Junya. They bury Marina’s body inside a gadget, and Takopi impersonates Marina, but the ruse quickly unravels. Meanwhile, Shizuka exploits Naoki’s guilt and anxiety, pushing him to the brink.

As Takopi’s erased memories return, he recalls an earlier timeline: he originally met Marina in 2022, not Shizuka. He had helped Marina escape her abusive mother, only for her to die in a tragic spiral of violence. That failure drove Takopi to travel back to 2016 with the intention of killing the young Shizuka – an act that violated Happy Planet law and caused his memory wipe. The full story of his original sin is revealed. In the final loop, Takopi sacrifices himself by reactivating the camera with his own energy, rewinding time one last time to before any tragedy occurred. In the new timeline, Shizuka and Marina become close friends, Naoki reconciles with his brother, and Takopi disappears, leaving behind only a drawing and the wish that they all find happiness. Years later, in 2022, the children have grown up together, their lives changed by the alien’s selfless act.