Over a drink at Quark's, a middle-aged alien widow, Alsia, tells an alien man, Martus, how she plans to invest her life savings in a large mining concession. Just as Martus offers to become her partner, Odo arrives and arrests him — for swindling a couple on the station. Martus is locked in a holding cell and meets a sickly alien named Cos, who laments that he lost everything he ever had because of a gambling device that always brought him bad luck. He activates the device and exclaims with surprise that he won — and then dies without warning.
Meanwhile, O'Brien worries to Keiko that he is getting old after Bashir literally destroys him in a racquetball game. Bashir also talks about the game with Dax, concerned that if he gives O'Brien the rematch he has demanded, O'Brien will have a heart attack. Later, Martus is released from his cell and heads straight for Quark's with the gambling device, with which he has been playing and winning consistently. Quark feigns disinterest in the device, but offers to buy it for a pittance. This is all Martus needs to realize it has value, and he heads for the Promenade with the machine. Within minutes, he has convinced a widow named Roana to let him turn her shop, which she is vacating, into Club Martus — Deep Space Nine's newest bar.
Furious at the prospect of competition, Quark begs Sisko to kick Martus off the station, but Sisko refuses. To add insult to injury, Martus takes on Rom as a minor partner. Later, Roana arrives at the club, and Martus toasts their joint success, feigning romantic interest in his new partner. He also shows her the gambling table, where several happy patrons are playing with replicas of the device he acquired from Cos. Quark's, on the other hand, is basically deserted, except for a very forlorn O'Brien, who has just lost another racquetball game to Bashir. His tale of woe gives Quark an irresistible business idea that will help him top Martus. Meanwhile, in Ops, Sisko notices that there have been several strange reports of bad luck on the station. Dax remarks that her luck has been especially good, just as several patrons of Martus's bar simultaneously hit the jackpot on the gambling device.
The next day, Quark summons O'Brien and Bashir to his bar, surprising them with the announcement that his tables are open for betting on a racquetball game they will play tomorrow. When he promises to donate half of the proceeds to charity, both men realize they cannot say no. In the meantime, the station's run of bad luck continues, and Sisko asks Dax to investigate. Martus's luck appears to change for the worst, when his bar is literally deserted before the big match. To make matters worse, Roana announces that she is shutting him down. Forlornly, he pushes the button on his gambling device, and comes up a loser for the first time. Still, he isn't ready to give up, and approaches Alsia with his profits, giving them to her to invest in the mining project.
During her investigation, Dax discovers that trillions of solar neutrinos aboard the station are not spinning in accordance with the laws of probability. She searches for a correlation as Bashir and O'Brien begin their match. O'Brien plays surprisingly well, and Bashir is unable to keep up. But O'Brien senses that something is wrong and stops the game. He throws the ball against the wall, and when it bounces back directly into his hand no matter how it's thrown, he summons Dax and Sisko. The two of them go to Club Martus, where they realize that the gambling devices are to blame. Unable to disconnect them, they destroy them with phasers over Martus's objections, and then Odo arrives to re-arrest Martus on the earlier swindling charge. Back in the holding cell, he encounters Alsia, who he learns was out to swindle him. But all is not lost. Quark arrives to bail him out, enjoying the fact that he once again has the upper hand.