The crew make final preparations for launch, under the watchful eye of the Captain Eleanor Stack. Heading out to the Med and the Black Sea, the men and women onboard face a heart attack, a man overboard, and a show of force from Russian airplanes.

The ship and its crew are deep in the heart of the Black Sea, heading towards Crimea, when a Russian ship HMS Duncan has been tracking suddenly disappears. The crew discover that the Russians are also tracking them and the situation soon escalates.

The mood on Duncan changes with the news that a chemical weapon is reported to have been used in Salisbury. When a second chemical attack takes place, this time in Syria, the ship goes to action stations for the very first time.

The crew stand-off with a Russian warship over Syria, and young sailor Danny tries for his first promotion, but chef Liam gets in trouble for drinking and might lose shore leave privileges.

The crew of HMS Duncan face their most important mission to date when they receive orders to join a French aircraft carrier carrying out airstrikes against ISIS forces in Iraq and Syria. For the crew, this means seven solid months at sea.

The crew must defend French aircraft carriers off the coast of Syria, and help evacuate Britains being held in Cyprus.