We all have or will have regrets in our lives, but Keisuke Higashi has had one he could never forget nor forgive himself for: letting his childhood friend and love, Cho Fujimori, die. Not wanting to live in that timeline without his love, Keisuke, along with his assistant Sakura Ashikaga, decides to dedicate everything to researching time. After countless trials and errors, they manage to build a time machine with the help of alien technology. Their successful voyage to another timeline, however, comes with an unexpected stowaway.