"The way is open! Enter and EXTERMINATE!"

Sub Zero - Episode 2

The Doctor begins to unravel the mystery of the 'doors in the ice', realising that is by no means the first time they have swallowed a ship in this area... Deciding they don't have a moment to lose if they are going to rescue the rest of the missing submariners, the Doctor and Lt. Davis procure a plane belonging to the base and head for the area where the sub disappeared.

Meanwhile, while they're away, 'ghostly' apparitions of what should be long dead sailors move against the unsuspecting research scientists, fooling them into opening the main doors of the base to allow their evil masters access so that their conquest of the Earth can finally begin...