"When It Rains..." is the 171st episode of the television series Star Trek: Deep Space Nine, the fifth of the final ten-episode arc of the series.
Plot
At a strategy meeting, Chief Miles O'Brien reveals that only one ship survived the previous Second Battle of Chin'Toka: a Klingon Bird-of-Prey that made a chance adjustment in its warp core which immunized it to the Breen's devastating energy dampening weapon. However, this fix cannot be applied to Federation and Romulan ships, putting the Allies at a severe disadvantage. Fortunately, the Dominion is putting its final offensive on hold in order to deal with Damar's uprising of the Cardassian military. To help Damar lead the revolt against Cardassia, Benjamin Sisko orders Kira Nerys to go with Garak to Cardassia to teach the Cardassians the techniques of guerilla warfare. Admiral Ross grants Kira a field commission in Starfleet, with the rank of Commander.
Julian Bashir's idea of using changeling tissue to create better synthetic organs for solids leads to his discovering that Odo is infected with the same disease plaguing the Great Link. Odo decides to go with Kira to Cardassia anyway. Bashir tries hard to get Odo's medical records from his visit to Earth three years ago, but when he receives a phony file he realizes that Section 31 created the disease and used Odo as a carrier.
After inducting Martok into the Order of Kahless, Gowron announces he will take a more active role in the conduct of the war. Gowron designs a plan to attack the Dominion, even though Worf and Martok point out that the Klingons would be outnumbered twenty to one.
Anjohl Tennan (Dukat) sneaks a look at the book of the Kosst Amojan and the Pah-wraiths punish him by blinding him. Winn throws Anjohl out (she was already aware of his true identity) and tells him he can return when his sight is restored.
Arc significance
Kira, Garak and Odo go to Cardassia to help Damar's resistance movement.
Gowron comes to the station to take a more active role in the war.
Bashir discovers that Odo suffers from the Changeling disease and that Section 31 infected him over three years ago (cf. "Homefront").
