The Changeling

Answering a distress call from the Malurian system, the Enterprise finds that its four billion inhabitants have all been killed. The ship is then attacked by a small probe whose energy bolts almost overwhelm the shields. When Kirk makes contact it stops firing and names itself Nomad. Spock identifies it as an Earth probe launched in the early 21st century and thought destroyed. Nomad mistakes Kirk for Jackson Roykirk, the scientist who built it, and is brought aboard. Damaged long ago, it merged with an alien probe called Tan Ru, and its mission has become a drive to find and destroy anything imperfect. It wipes Uhura's memory, kills Scotty and then restores him, and kills two guards. When Nomad decides the whole crew is imperfect and must be eliminated, Kirk points out that it has made mistakes itself, not least in calling him its creator. Unable to accept its own imperfection, Nomad is beamed into space and destroys itself.