In a
parallel world, seven-year-old Koyomi chooses to live with his father Shodai Hidaka following his parents' divorce. At Imaginary Science Research Institute, Koyomi listens to his father explaining Imaginary Science, the study of parallel worlds, and Imaginality, the individual's consciousness that shifts between worlds called Parallel Shift. He is then informed that his grandfather's dog, Yuno, has died in an accident. Shiori Satō, daughter of the Institute's director Genko, finds him crying, so she brings him to an IP capsule that is capable of bringing an individual to parallel worlds. Koyomi shifts to a world where Yuno is alive but also where his grandfather has died. Returning to his original world, Koyomi learns that Shiori has helped him in exchange for his help in operating the capsule and sending her to a world where her parents are not divorced, but Genko stops them. Since then, Koyomi and Shiori become close friends as they grow up together. Koyomi and Shiori are called up by their parents, whom they find discussing the Imaginary Elements Print (IP), a pattern that can determine how far an individual is from the parallel world they have arrived to their original one numerically, and the Optional Shift, a study on a forced Parallel Shift to a specified parallel world. Shodai and Genko stop their discussion to announce their marriage to each other. This causes uneasiness between Koyomi and Shiori, who both realize their love for each other, and they decide to do Parallel Shift to a world where they are not going to be stepsiblings. They successfully arrive in a world where their parents are not divorced, but Shiori dies after getting hit by a car at the Showa-dori intersection. Koyomi returns to the original world and finds Shiori unconscious. Shiori is proclaimed
brain dead while her heart is kept beating artificially. Koyomi then finds Shiori's Imaginality in the form of a ghost at the intersection. Shodai and Genko explain that the time of death of the alternate Shiori's body and the time of shifting of the original Shiori's Imaginality back to the original world have overlapped, causing the latter's Imaginality to lose sight of her original body. Koyomi expresses his desire to save Shiori, so he helps in the experiments about Parallel Shift while keeping on attending high school. At the Institute where he is now employed following his graduation, Koyomi learns that Shiori's heart has stopped working. Sometime later, Kazune Takigawa joins him as his assistant. After they visited a
karaoke, Koyomi finds Kazune also capable of seeing Shiori's Imaginality at the intersection. Koyomi later reveals that he is researching the Time Shift to send Shiori to a world outside of her event radius, which refers to her accident. Kazune agrees to help with his research. A decade later, Imaginary Science has advanced significantly and the Optional Shift becomes a reality. Koyomi still fails to make the Time Shift possible but after hearing Kazune's mention of the
buoyancy and
viscosity of a beer, he takes these concepts to solve his research. Koyomi tells Kazune that he plans to shift Shiori back in time to a
parallel world where they do not meet. He mentions that Shiori is
entangled with him, so he will also do the Time Shift with her which will result in his brain death. Decades later, old Koyomi prepares for the Time Shift with old Kazune, who has agreed with his plan despite showing hesitation in the past. Koyomi says goodbye to Shiori and explains that going back to a time when they go to their separate lives will help her get out of her event radius. He promises that they can meet again at the intersection on August 17 at 10 AM when they grow old. Kazune begins the Time Shift after hearing Koyomi's final goodbye. In a
post-credits scene, old Koyomi Takasaki of a parallel world is encouraged by his wife Kazune to show up at the intersection. He later encounters old Shiori there and asks for her name. ==Voice cast==