"The Adversary" is an episode of Star Trek: Deep Space Nine, the season finale of the third season. In it, a Changeling impersonates a prominent ambassador in an attempt to hijack the Defiant and start a war between the United Federation of Planets and another Alpha Quadrant power.
Commander Benjamin Sisko is promoted to Captain in this episode.
- This episode introduces the idea of blood testing to tell who is or isn't a Changeling. At one point Sisko points out that several people in a certain setting could be a Changeling, including himself. But Odo points to a bleeding wound on Sisko's hand. Humanoid blood, once it leaves the body, is dead tissue; "blood" leaving a Changeling's body would revert to shapeshifting jelly. This suggests a simple effective way to test a person suspected of being a Changeling. The idea is reminiscent of John W. Campbell's classic novella, "Who Goes There?", made into two movie versions both called "The Thing", in which scientists at an Antarctic weather base devise a similar test to detect who has or hasn't been infected by an alien virus that can perfectly mimic other life forms. Sisko's own father would point out a possible flaw in the test in the episode "Homefront".