Baines was a security guard at a research facility.
He and the other guards tried to stop the Seventh Doctor and Mel from entering the facility, but Dr Leech let them stay.
When it started raining jellyfish, Baines and the other guards left the facility to deal with the situation. Mel learned that salt defeated the jellyfish, and passed the information on to Baines. (PROSE: The Invertebrates of Doom)