“Second Skin” is episode five of season three of Star Trek: Deep Space Nine.
Major Kira is contacted by someone from the Bajoran archives related to the time she spent in a Cardassian prison camp. Only Kira has no memory of this, as she knows she was elsewhere. Yet the only surviving fellow inmate from the time also remembers her being there, so she goes to Bajor to find out what happened.
Sisko is then contacted by the same woman who contacted Kira, saying she never arrived. Indeed, she didn’t – she’s been abducted.
Kira wakes up on Cardassia Prime, and she now looks like a Cardassian. A member of the Obsidian Order tells her that she, too, belongs to the order, is a Cardassian, and is a deep cover agent who was sent to Bajor in the person of a dead resistance fighter, with both her appearance and memories adjusted, and now she’s been brought home.
Kira refuses to believe this, of course, but there’s a Cardassian Legate who believes she is, in fact, his daughter. She’s been given something to bring her memories back, but this doesn’t work. Kira is also shown a dead body of a Bajoran who looks like she did, and eventually her faith starts to crumble a bit.
Meanwhile, Sisko is taking the Defiant to Cardassia to find her, and takes Garak along to help.