Gaku Mashio's father, Harumi, died. On the night of his father's wake, Gaku went to go fish on the breakwater to reminisce about him, only to fall into the water instead. When he came to, he'd slipped back into the past 22 years. Not only that, the person who had fished him out of the water was a 16 year old Harumi.

(Source: Akita Shoten, translated)