This episode from Doctor Who: The Sixth Doctor Adventures: Water Worlds is available free to Big Finish Newsletter Subscribers in May 2022.
The Sixth Doctor returns, and he's thrown straight in at the deep end! Travelling the galaxy with Melanie Bush and their brand-new companion, marine biologist Hebe Harrison, there are wonders to see, dangers to face and plenty of peril beneath the waves.
1.1 The Rotting Deep by Jacqueline Rayner
A mysterious SOS summons the Doctor and Mel to an oil rig in the North Sea where a dwindling group of survivors awaits rescue from a lethal menace. One of their number is Hebe Harrison, a wheelchair-using marine biologist who is definitely more than she seems. Can our heroes escape the rig? And just what is killing off the rig's beleaguered crew?