Duet (Star Trek: Deep Space Nine, Season 1, Episode 19)
Major Kira Nerys realizes that a passenger who came to Deep Space Nine to be treated for a rare disease was, years earlier, involved with the occupation of her planet. She has to figure out who he really is -- and then figure out what's going on when he suddenly admits to being a war criminal who slaughtered thousands of her people. Ultimately, she realizes she has to re-examine her prejudices about the Cardassians.
Watch Star Trek: Deep Space Nine on CBS All-Access.