Itou Kaiji is a bum who steals car emblems and slashes tires on a regular basis. However, one day, this routine changes when a man in a trench coat pays him a visit. The man reveals himself as Endou, a debt collector, who has come to claim an unpaid loan of 300,000 yen, which Kaiji had previously co-signed with his co-worker, Furuhata Takeshi.

Because of Takeshi's disappearance, Kaiji is left with an overwhelming balance, but is given the chance to gamble against other bankrupt bums aboard a boat — each gambler is loaned additional money, giving them a chance to pay their dues or become in even deeper debt.

Kaiji accepts the challenge in hopes of repaying the compounded debt of 3,850,000 yen and, with luck, winning additional cash...

[taken from AniDB]