In a lab on an alien cruiser, Bashir technobabbles a container of orange goop, turning it green, which we are made to understand is good. The T’Lani and Kellerun races have recently made peace after warring for centuries, and the script required them to reach out to the Federation for help destroying their stockpiles of weaponized ooze.
O’Brien wants to go home after being away for a week, but Bashir talks him into staying one more day for the peace celebration.
Just as Bashir finishes de-weaponizing the last of the orange goop, two Kellerun soldiers storm the lab and start shooting the T’Lani scientists. Something is blocking our heroes from beaming away to their runabout, but O’Brien and Bashir do manage to beam down to the surface of the planet they are orbiting.
On DS9, ambassadors from the two races bring Sisko the sad news that O’Brien accidentally triggered a security device that killed everyone in the lab. They provide a security recording so Sisko can see for himself.
In a bunker on the planet surface, O’Brien and Bashir reason that they must warn the T’Lani that the Kelleruns have betrayed the peace treaty. Eager to help O’Brien fix a comm system, Bashir mentions the “engineering extension courses” he took at Starfleet Medical, but O’Brien blows him off.
Sisko watches a recording of something we know did not happen — O’Brien saying that he accidentally triggered a security program. Kira is surprised that O’Brien didn’t detect it, and furious that the T’Lani didn’t warn him about it.
As O’Brien fiddles with the tech, Bashir, bored, chatters about how attractive T’Lani women are, and sympathizes that because O’Brien is married the most he could have hoped for at the celebration banquet is a good meal. His continued nattering about how the adventurous life of a career Starfleet officer is too dangerous for a family annoys O’Brien even more.
Soon we learn O’Brien has been infected by the orange goop. A bit later, he is too sick to keep working, and talks Bashir through further tinkering.
Sisko brings the bad news to Keiko, who asks to watch the surveillance tapes.
Dax tells Kira that Bashir gave her his med school diaries so she could understand him better, and is sad that she never got around to reading them. Quark offers drinks on the house to the memory of Bashir and O’Brien, and gives a commerce-centric but nonetheless sincere toast.
Because this is TV, Keiko has noticed the security recording, time-stamped in the afternoon, shows O’Brien drinking coffee, something he *never* does in the afternoon. Sisko promises to investigate what appears to be a falsified record.
O’Brien continues to fade. He asks Bashir to tell him about the woman he fell in love with — a ballerina whose father offered him a good job, but he chose Starfleet instead. Because this is TV, immediately after this touching speech that humanizes the vain and needy Bashir, O’Brien reports he can no longer feel his legs.
Sisko and Dax go to the T’Lani system, ostensibly to pick up the abandoned runabout, and start a low-key investigation.
In the bunker, O’Brien instructs Bashir how to tweak the comm system, and also gives a touching salt-of-the-earth speech on the theme “marriage is the greatest adventure of them all.”
On the abandoned runabout, Dax finds evidence that someone requested a remote transport after the accident on the cruiser is supposed to have killed O’Brien and Bashir.
The big twist comes when we realize it’s not the Kelleruns betraying the T’Lani, but rather both leaders working together, with the goal of eliminating all people capable of re-creating the orange goop weapon — and that includes O’Brien and Bashir.
When it looks like the jig is up, O’Brien says it’s been an honor serving with Bashir. We know full well these two leading characters won’t die, but there are some enjoyable character moments and plot twists before the actual happy ending.
Recovering in the infirmary, O’Brien hastily interrupts Bashir’s heartfelt speech about the bond that forms when two people face death together. Keiko is grateful Bashir saved his life, but O’Brien grumps that “It was hell” to suffer through Bashir talking all the time.
The final plot twist — a revelation about O’Brien’s habits — is unexpected and clever.