Stardate not given: The Enterprise is sent on a mission back to Earth in the year 1968 to discover details about how the planet survived the arms race. While in orbit, the ship intercepts a transporter beam from an unknown part of the galaxy and beams the space traveler aboard. Surprised by what has taken place, the man identifies himself as Gary Seven and claims to be a 20th Century Earthman raised on an unknown world and trained to prevent Earth from destroying itself. Kirk decides to verify Seven's story before releasing him, but Seven escapes and beams down to the planet below. Kirk and Spock follow him to New York City, and Seven meanwhile discovers that two of his fellow agents have been killed in a auto accident. Seven is forced to complete their mission himself which is to sabotage an orbital nuclear platform, just low enough in the atmosphere to scare Earth leaders into prohibiting additional nuclear space weapons. Just as Kirk and Spock arrive, Seven beams out of his office a

The Doctor, Jamie and Victoria arrive at an outpost drilling for natural gas in the North Sea. Mysterious heartbeat sounds are echoing through the pipes, there have been drops in pressure, and contact has been lost with several offshore rigs. Victoria is attacked by a monstrous form of living seaweed, and some of the base personnel are possessed by the poisonous Weed, which stings them and draws them under its control. As the Weed steps up its attack on the base, the Doctor discovers that it's vulnerable to high-frequency sounds, and drives it off with the help of Victoria's screams - amplified by his trusty new invention, the sonic screwdriver. However, Victoria has had enough, and she chooses to remain on Earth, albeit a century after her own time.