Miss Young worked for the BBC in a secretarial role. She invited the Third Doctor to a party being thrown by her employers, which he attended after investigating a missing film crew. (PROSE: Countdown to TV Action)
On one occasion, she went to Scotland with the Fourth Doctor, in Bessie to meet with Professor McBane, an old friend of the Doctor who had summoned him. The friend revealed that a meteor had crash landed in oil rig waters, and he wanted the Doctor's input on the chance of alien hostility. As the group flew over the area in a helicopter, giant tentacles arose from the sea and began attacking a local ship. The Doctor was able to take a sample of the pink substance from the creature back to the laboratory, where he discovered that it was seaweed contaminated from radiation from deep space brought by the meteorite. Furthermore, the mutated seaweed was attracted to sound, so the sound of the oil rig put it in danger.
The Doctor and Miss Young headed out on a boat, using a sound-emitting gadget to attract the mutated seaweed. A tentacle grabbed the Doctor, but Miss Young lashes out and cuts it off with a large knife. They lured the mutation into a shallow bay where it couldn't dive and dropped drums of petrol from the boat. After they were flown out of the area, the petrol was set on fire, killing the beast. Afterwards, the Doctor and Miss Young drove back to the TARDIS for more adventures. (COMIC: The Sea Devil)
Behind the scenes
Miss Young was never identified with a first name in any of her appearances, and was a comic-exclusive "filler companion" created by TV Comic — apparently as a replacement for Leela, who had previously been featured in the 1977 TV Comic stories. TV Comic Annual 1979 was published in September 1978, by which time Louise Jameson had left the series, but it is also worth nothing that most of the TV Comic stories from 1978 feature the Doctor travelling alone.
This implies Miss Young was either created due to lack of information about the future of the show, or because TV Comic did not want to pay extra licensing fees for the use of the Doctor's new television companion Romana, as played by Mary Tamm — which is similar to why the original TV Comic companion characters of John and Gillian were created.
However, this doesn't explain Miss Young's Leela-style dialogue at two points during The Sea Devil — namely "Let us hurry, Doctor. If the evil thing should return..." as the Doctor takes a sample of the mutated seaweed from the sea; and "Back, sea devil!" as she lashes out and cuts off with a large knife the tentacle that grabs the Doctor.