Kurosaki intervenes and tells Ōtsuki that he has no choice but to play, and in the end, he loses 18,252,000 perica which Kaiji splits equally between the Forty-fivers. Kaiji then asks Kurosaki to approve their requests for the one-day outside pass and he agrees. The Forty-fivers insist that Kaiji should keep the money and twenty one-day outside passes, leaving him with ¥800,000 (around US$7,800) to gamble for the team and pay off their debts. Now back in society above ground, Kaiji meets Sakazaki Kōtarō who is looking for a partner to win big in the illegal casinos. Kaiji accepts his offer, and is taken to an illegal casino where he sees a huge Pachinko machine called "The Bog". Each ball is worth 1,000 times more than normal and the jackpot is worth ¥550 million, more than enough to free Kaiji from debt.

When Kaiji is told there have only been two winners of the Bog, Hyōdō and Tonegawa, he realizes that the game must be rigged. He then agrees to work with Sakazaki for 10% of the possible ¥600 million payout from the Bog, however Sakazaki is secretive about the details of his plan which includes the machine's maintenance schedule. A day before their attempt to beat the Bog, Sakazaki appears with ¥50 million he 'borrowed' from the company president's safe where he works as a security guard. At the appointed day, Sakazaki commences to play the Bog.

Sakazaki starts to play the Bog, using an initial ¥10 million, dropping balls onto the final holed plate, but they do not have enough momentum. During the game Kaiji successfully passes Sakazaki his pre-prepared magnetized beer can and he prepares to use it to increase the centrifugal force of a ball to make it drop through the winning hole. However the effect is insignificant, but Sakazaki continues, wagering another ¥10 million. Kaiji overhears the security men talk of a device to prevent anyone winning and Kaiji warns Sakazaki's to stop, but he is in a gambling frenzy and continues to play using the money he 'borrowed' from the company safe.

Sakazaki ignores Kaiji's warning and continues sinking money into the Bog, while the casino manager gives instructions to initiate the 'block' which controls the flippers, stopping the balls descending into the winning zone. Although Sakazaki is close to winning he collapses and the casino manager, Ichijō Seiya, introduces himself and reveals that they are using brass balls today which are not affected by magnetism. They leave with a little more than ¥6 million after losing over ¥43 million in two hours. The next day, Kaiji spends hours back at the casino, staring at the machine, then goes to the staff room to cash in one ball he found, insulting Ichijō but also gathering information about the casino's operations.

Kaiji returns to the Sakazaki apartment, only to find it trashed and empty. Sakazaki has taken the last of his money to the race track. Kaiji's plan for revenge involves Sakazaki, so with a plan to beat the Bog, he tracks Sakazaki down at a Sapporo racetrack. Kaiji approaches Endō who is now broke but agrees to fund him, however it will cost ¥113 million including interest plus 50% of the winnings. Sakazaki violently objects but they end up agreeing to split the winnings three ways after costs.

Sakazaki enters the casino to challenge the Bog again, dressed as a Komusō monk. He plays the machine, quickly losing his stake of ¥6 million, then attacks and damages it. Ichijō is suspicious of his behavior and realizes that it is a distraction and rushes to his office to find Kaiji there. Ichijō has Sakazaki beaten up and Kaiji tortured, then they are thrown out into the street. Days later, the Bog is repaired and operational again. It is Kaiji's final day on the surface and he arrives at the casino to challenge the unbeatable Pachinko machine.

On Kaiji's last day above ground, he challenges the Bog and Kurosaki warns Ichijō that Kaiji must have a plan, and Ichijō's reputation is on the line. Kaiji starts playing and more balls than expected drop between the pins and Ichijō is suspicious. It takes him some time to realize that Kaiji somehow entered the staff room and doctored the setting gauges for adjusting the pins so they were set wider apart. Panicking, Ichijō has the 'block' turned on, directing the balls away from the winning path.

Kaiji plays on even though it appears the chances of winning are zero. Meanwhile, Ichijō receives a request to send a live CCTV feed to the Teiai underground forced labor camp and to Hyōdō. Kaiji continues to play, even after losing ¥20 million in what seems a pointless exercise.

Kaiji continues to play, with every ball deflected by the flippers until one ball makes it through to the plates below, the flipper rods bent because the replacement rods installed after Sakazaki smashed the machine were modified by Kaiji. Ichijō starts to panic and then receives a phone call from Hyōdō telling him to let the game continue and not bring the corporation into disrepute, under penalty of death. Ichijō now relies on his three rigged bottom plates to foil Kaiji's chances of winning.

Kaiji has only ¥20 million left to try and beat the Bog, however he is confident as he is aware of how the three bottom plates are rigged. He explained this to Sakazaki when they were at the racetrack. Slowly Ichijō comes to realize that Kaiji managed to not only tilt the machine to increase his odds of winning, but also create a slightly favorable tilt on the casino building. Unbeknownst to him, Kaiji's group had poured 20 tons of water into plastic tanks in a room on one side of the casino building, causing it to subside slightly on that side.

Kaiji's improbable idea of tilting the building to tilt the Bog pays off, and he gets closer to winning with more balls dropping onto the third and final plate. Ichijō contemplates the consequences of losing everything he has and tilts the machine again in the same direction to prevent balls reaching the third plate as Kaiji uses his last ¥10 million.

Kaiji plays on, but he is on the verge of defeat, having lost all his money. He convinces Endō to lend him his last ¥10 million against his better judgement, but at an exorbitant interest rate, and Kaiji returns to the machine for a final assault.

Even with the plates filled to the brim with silver balls, Ichijō's secret weapon, the Curtain of Wind, prevents Kaiji from winning and Kaiji has run out of money. As he is dragged away, Sakazaki appears swearing vengeance on the machine, and gives Kaiji another ¥10 million which he again 'borrowed' from the company safe. Kaiji starts playing again with renewed vigor. The bottom plate is now awash with balls, but none can pass the curtain of wind around the winning hole. The machine appears to groan, and with the plate overflowing with balls, some cascade down the winning hole. Horrified that Kaiji won, Hyōdō sentences Ichijō to 1,050 years underground.

As Ichijō is removed by the men in black, Kaiji tells him to fight his way back to the surface for a rematch. Meanwhile, Kaiji's friends cash in the balls he won for a total of over ¥729 million. Kaiji, Sakazaki and Endō settle their debts and split the remainder for almost ¥190 million each, from which Kaiji must pay back his friends in the underground labor camp.