A Ukrainian paramilitary group makes a series of terrorist attacks against the UK's Special Boat Service to weaken their operational capability and facilitate the theft of a new British ballistic submarine armed with tactical nuclear weapons. The stolen submarine threatens Russia with nuclear attack seeking the repatriation of the Crimea to Ukraine. Major Sam Brent and Captain Jack Tomkins are drawn out of retirement and whilst seconded to MI5 also embed in special forces operations to assist in retrieving the submarine and bring the terrorists to justice.