Dr Leech was a researcher at an institute attached to a nearby university.
He was examining an artefact that had recently been brought to him by several university students who had found it in a cave. He was making no progress, and was beginning to suffer from exhaustion. He suspected the artefact was the cause, as the students who had discovered it had been put in the medical wing for exhaustion.
Leech was unaware that his assistant Ly, who had been part of the exploration party, had come under the influence of the artefact and was putting zooplankton in his drinking water.
When the Seventh Doctor and Mel arrived, Leech welcomed their help. A Cnidarian ship appeared, and it began raining jellyfish. The artefact showed a hologram of the Cnidarian commander, who then grabbed the Doctor and brought him aboard the ship. Leech then collapsed.
Mel and Ly brought Leech to the medical wing. Ly then revealed what she had done, and Mel realised that Leech and the eight students were growing tentacles. She also discovered that they had a negative reaction to salt. Leech and the students were cured by receiving saline drips. (PROSE: The Invertebrates of Doom)