It is written by Victor Pemberton, and stars Tom Baker as the Fourth Doctor, Elisabeth Sladen as Sarah Jane Smith and Bill Mitchell as Zor.

[citation needed] The Doctor and Sarah Jane arrive on a beach by the Thames Estuary at night, and discover a metallic seaweed there.

The Doctor consults with Professor Emerson, who says that three expeditions to recover a recent meteorite from the bottom of the estuary have all vanished.

The Doctor recalls his visit to planet Pesca, where he fell down a chasm into a cavern and met Zor, the Pescaton leader.

Back in London, Sarah Jane finds an abandoned baby while the military shoot rounds at the invading Pescatons with no effect.

Professor Emerson helps the Doctor build a high frequency sound trap inside a sewer, and Zor is lured there to be destroyed as the planet Pesca disintegrates in its orbit.