The Fires of Vulcan
Mel has encountered UNIT previously (Business Unusual). She built a replica Mount Vesuvius at primary school and learned Latin at some point.
"'I've seen the future, Mel. I know what will happen - What must happen.'"
The TARDIS arrives in Pompeii, where the Doctor knows the TARDIS is destined to be buried by the eruption of Mount Vesuvius. Time cannot be cheated - the TARDIS must be buried, and he and Mel must be trapped there.
The first audio adventure featuring Melanie Bush is excellent - easily making up for Head Games! Despite the Doctor acting more like he did in later seasons, it fits with the story, and the fact he knows what the future holds for Mel and himself, just for this story...