The crafting of an imposter

I suspect there might be a "Star Trek: Deep Space Nine" episode that I haven't seen yet.

I am thinking of a story-arc involving Odo and Kira, which I recall coming to a focus in a cave where Odo could distinguish an imposter from the purportedly-real Kira by the fact that the purportedly-real Kira would never admit to such a thing as loving Odo.

I do not recall any clues in that episode indicating that the audience, or even the author(s), were to be taken as being aware of a mechanism by means of which whatsoever is believed to be true is perceived as true, thus the impression I took from the episode was that the creature Odo was observing was in fact an imposter, rather than its having actually been the purportedly-real Kira up until the moment when his belief that it was not crystalised into the material plane he was experiencing, resulting in the purportedly-real Kira being replaced (possibly by a switch of timeline or something, who knows, there could be many potential mechanisms for such things for all we know, maybe?) by an imposter.

So there you have it, that was the highest point of that arc I recall having seen. The reason I suspect the existence of an episode that I have not seen is that I have since seen more than one episode in which, presumably, Kira has been permanently replaced by some such imposter, as witnessed by the fact that, lo and behold, the creature purported to be Kira expresses expressions that presumably correspond to the kind of expressions Odo had believed, in the cave, the 'real' Kira would not express.

Hmm. I kind of wonder how it came to pass that Odo's timeline changed from one in which the real Kira would not express such things to one in which she would. :)

~mgm~