Ly was a university student and the assistant of Dr Leech.
One day she led seven other students in exploring a cave. They found a Cnidarian artefact that absorbed their life forces. When they returned to the university with the artefact, the other seven were placed in the medical wing, suffering from exhaustion, but Ly returned to her work with Leech, studying the artefact.
While working with Leech, Ly put zooplankton in his drinking water. The Seventh Doctor and Mel arrived, tracking an anomaly that drew the TARDIS off course. A Cnidarian ship arrived, and Commander Hydra Sowerbii brought the Doctor aboard.
Leech became weaker. When Mel and Ly brought him to the medical wing, they saw that the other students were growing tentacles. Ly then admitted that she was part of the exploration party too, and she started to advance on Mel. Mel threw a saline bag at her, and Ly screamed.
Removed from the Cnidarian influence, Ly asked to be brought on board the Cnidarian ship. She opened a pack of table salt in the ship, contaminating the fresh water. The Doctor and Ly were freed, and the Cnidarian ship left.
Ly and the other students were cured by receiving saline drips. (PROSE: The Invertebrates of Doom)