Giant submarine tankers have been a dream in the oil industry for years. Now, they are not only possible but necessary-to ship oil out of Alaska under the ice.

Mogul's experiments have gone well. Now Stead, Thornton, and Izard visit their submarine - not in Alaska but in the Caribbean. Even there, though, things can go dangerously wrong - as Stead finds on his first underwater dive.

Stead, Thornton and Izard are on trial tonight. State secrets have been leaked to a foreign power - and the Ministry of Defence investigator says it must be from one of these three men.

In this final episode, the action all takes place in Mogul House as Commander Fayne (Patrick Allen ) conducts his interrogation.