After leaving Happy Planet on a mission to spread happiness across the universe, a pink squid-like creature lands on Earth. Things look bad when it barely avoids starvation and getting caught, until one day, gloomy fourth-grader Shizuka Kuze decides to help the strange life form and names it Takopii. As Shizuka comes back daily to feed it, grateful Takopii endeavors to pay her back by making her smile.

However, that is not an easy feat—Takopii is oblivious to the complexities of human society and the fact that Shizuka's life is filled with hardships. No matter how desperate the situation, Takopii is always cheerful and determined to make Shizuka feel better by utilizing different methods and gadgets originating from Happy Planet, even as his mission begins to spiral out of control.