The episode begins with Odo placing El-Aurian Martus Mazur in a holding cell after accusing him of swindling aboard Deep Space Nine. While in his cell, Martus acquires a gambling device that alters the laws of probability after his cellmate, an old man named Cos, dies. Because of his change in luck, he is released from confinement after the elderly couple he conned decides not to press charges.
With his new and intriguing device, he sets up a bar and casino that ends up drawing business away from Quark. However, the device soon turns on him and he himself is conned into losing a large sum of money. In addition, the couple he swindled out of money changes their mind and presses charges against him. To prevent more tampering with natural probability, Sisko and Dax destroy the device.
A separate but related story involves O'Brien's attempt to get in shape so that he can compete effectively with Bashir at racquetball. As part of a ploy to regain lost business for the bar, Quark is able to trick O'Brien and Bashir into participating in a public tournament that people can bet on. When O'Brien performs abnormally well, he realizes that there are statistical abnormalities on the station. This leads the crew to discover the effects of the devices in Martus's establishment, and O'Brien and Bashir agree to cancel the rest of the game.