Ed and Beckett go 'under cover' and end up on board a submarine. They are, however, found out to be fakes and, whilst Ed escapes, Beckett is left on the submarine with a money hungry man, eager to gain access to a bank and steal some diamonds a long the way. Beckett ends up getting trapped on the submarine, as it slowly fills with water. The man, meanwhile, has got his associate to steal these diamonds, but their plan is foiled when Ros enters the scene. Ed goes under the sea to search for the submarine and but will he find Beckett in time?