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.