Itou Kaiji is a bum who steals car emblems and slashes tires on what seems to be a regular basis. This routine changes one day when he is paid a visit by a man in a trench-coat. Once the two get talking, it seems that the visitor (Calling himself Endou) is a debt collector. The reason for his visit is an unpaid loan which Kaiji had previously co-signed for a work-mate (Furuhata Takeshi). The original loan was 300,000 yen and once Takeshi had disappeared, the loan then fell on Kaiji.
Kaiji is then told of a way to clear the interest compounded debt (which stood at 3,850,000 yen), which involved getting on a boat with others in his position. Once on the boat the debtors would then have to gamble with loaned money, which would end with a few winning, and others getting into deeper debt and having to work to pay off their debts. After some coercion Kaiji accepts a position on the boat, in order to clear his debt and make a bit of money as well...
— Description from AniList community (anilist.co).
