Chief O'Brien is just about to head off on vacation with his wife Keiko when he runs into an old crewmate named Boone. The two had served together on the U.S.S. Rutledge, before O'Brien served aboard the Enterprise and long before he arrived on Deep Space Nine. As he greets Boone he does not notice that the man is recording his voice.
The O'Briens set off for their trip, but do not get very far before their runabout is stopped by the Cardassians and Chief O'Brien is arrested. Back on DS9, Commander Sisko is informed that O'Brien has been declared guilty and sentenced to death, and will have a ceremonial "trial" in which the verdict is read. He will then be executed. So far, no one has been told what the charges are.
Meanwhile, the crew of the station discovers that a number of warheads have been stolen from the station and have probably ended up in the possession of the Maquis, a faction that has been attacking the Cardassians. Major Kira looks into the station's records and finds that the transfer of the warheads was made by Chief O'Brien. She plays a security log containing O'Brien's voice, and in it he clearly requests access to the warheads. This evidence will be used by the Cardassians to demonstrate his guilt before his execution.
Dax and Dr. Bashir take the security log apart and are convinced that O'Brien's vocal command is a simulation. A voice sample was altered into a message that O'Brien never actually said. When this new evidence is sent to the courtroom on Cardassia Prime, the judge dismisses it.
Sisko interrogates Boone, the stranger seen talking to O'Brien just before he left the station. When Bashir examines him, he finds that the man is not the real Boone but a Cardassian surgically altered to look like him. His appearance was altered so he could get close enough to O'Brien to record his voice. With little time left before the end of the ceremonial trial, Sisko arrives in the courtroom with "Boone". The judge sees that the game is up, and releases O'Brien.