Gaku Mashio was a first-year high school student who lived with his father. He lost his father in a sudden accident, just as he got suspended from school after getting into a fight. On the night of his father's wake, Gaku went fishing on the shore in memory of his father, but he fell into the sea and was returned 22 years into the past. Gaku's father, Harumi, who was the same age as Gaku at that time, saved him from the water. Harumi's parents (Gaku's grandparents) took a liking to Gaku, and accepted him into their family under the name Gaku Miyagi. His grandparents even encouraged him to go to Seiha High School, same as his father. Twenty-two years ago, the town of Kurei was full of delinquents. Now Gaku and his father will fight their way to the top of the town!
(Source: Shonen Champion, translated)
— Description from AniList community (anilist.co).
