Like many in her class, Koyuki Honda looks forward to her graduation ceremony. However, while her peers have family to attend, Koyuki has no one, as her only family—her brother—died years ago. While visiting his grave, she meets two brothers named Kiri and Victor Putin chasing a strange creature. They try to assure her that they are just actors shooting a movie; Victor, however, piques Koyuki's curiosity about the old legend of the clock tower at the cemetery: if you make a wish under the clock tower at midnight during a full moon, time will turn back, allowing someone to reclaim what they have lost.
Deciding to make her wish, Koyuki learns the horrifying truth: the legend is a ruse for time demons called "Horologues" to steal humans' time. As they are drawn to regrets, Victor uses Koyuki as bait to lure a demon there. Kiri and Victor explain that they are Chronos Rulers, warriors who fight against the Horologues using the "Oath of Time"—an item allowing them to speed up or slow down time.
With the demon destroyed and the crisis averted, Koyuki is able to go on her way free from regrets. However, she is not the only human with sorrows. The two boys must continue to hunt down Horologues in order to keep humanity safe.