The story takes place in the near future, where the sea level has risen rapidly for unknown reasons, submerging most of the land. A boy named Banjiu Xiasheng, who lost a leg in an accident during childhood, tired of urban life, moves to a seaside village. All that his grandmother, who was once a marine geologist, left him were a boat, a submarine, and a pile of debts. In order to reclaim the 'lost future,' Xiasheng dives into the sea with a mysterious debt collector, Catherine, to find an underwater warehouse that supposedly holds his grandmother's legacy. There, he encounters an incredible girl named 'Atori,' who is asleep in a coffin-like device. She is a robot with a structure so intricate that she is indistinguishable from humans, and she possesses rich emotions. After being salvaged from the seabed, Atori says, 'I want to carry out the last command left by my master. Before that, I will become Mr. Xiasheng's leg!' In a peaceful town that is gradually sinking into the sea, the unforgettable summer of the boy and the robot girl begins.This work is adapted from the same-named game 'ATRI -My Dear Moments-' produced by Front Wing and Makura, which has recently announced an anime adaptation.