Odo arrests Martus, a man from a race who has the ability to get people to tell them things, apparently. Meanwhile, O'Brien's built a racquetball court, and it turns out that Bashir is highly experienced at it. O'Brien gets his arse handed to him.
Martus's cell mate, Cos, says he lost all of his money because of a gambling device, which he claims is bad luck. Cos plays the device, finally wins a game, and promptly karks it.
O'Brien slinks back home with his tail between his legs and complains to Keiko who suggests he's getting old (way to be supportive there). Meanwhile Bashir tells Dax that he was worried that O'Brien was on the verge of a heart attack all through the game.
Odo lets Martus out, and he pops over to Quark's bar to get insulted. Martus now possesses the gambling the device, which seems to like him. Quark attempts to buy it off him without success, and Martus goes and acquires a bar further down the Promenade...
O'Brien and Bashir play racquetball again, with Bashir very transparently letting O'Brien win, much to the latter's disgust.
Quark complains to Sisko. At length. Then Martus poaches Rom. Martus' casino is doing extremely well, and O'Brien finds Quark's bar almost deserted. Sisko observes that there seems to be a plague of bad luck on the station of late.
Quark lures customers back to his bar with promises of a racquetball game - Bashir and O'Brien are unimpressed, but when Quark says he'll donate the takings to a Bajoran orphan charity, they're obliged to co-operate. Quark has his customers back, while Martus laments about his sudden run of bad luck and is kicked out of his casino.
O'Brien and Bashir prepare for the racquetball match, and Quark tries to dope Bashir to fix the match. Quark explains that all the money is on Bashir, so if he wins, there won't be any profit for the orphans - Bashir says that if the orphans don't get anything, he and O'Brien will be very annoyed.
Dax reports that the laws of probability are out of whack. Because that's actually scientifically possible. Meanwhile, Rom quits on Martus. O'Brien and Bashir start their game, and Bashir seems to be having extremely bad luck. O'Brien stops the game and they discuss what's going on. After some scientific experimentation, O'Brien discovers that he can throw the ball anywhere and it will return to his hand.
Dax and Sisko track the problem down to Martus' giant-sized replications of the gambling device. The devices are quickly destroyed, and Odo turns up to take Martus back to a cell. Quark turns up to gloat. The end.
How infinitely improbable.