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

HMS Northumberland is ordered to investigate when Navy Command sends news about a gathering of Russian warships and submarines off the coast of Ireland. The timely arrival of ship's new chaplain coincides with a growing problem of decreasing morale, brought on as a result of the crew's inability to take time off. As HMS Northumberland leaves port, the chaplain tries to help the crew through another challenging mission.

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.

HMS Northumberland hunts for a Russian submarine in the Arctic Circle after the vessel disappears from the ship's radar. While the search continues chef Sauhil is feeling the strain below deck as he helps cook dinner for the crew of 200, but the preparations are interrupted by a serious incident as poisonous fumes seep into the upper deck

The crew make final preparations for launch under the watchful eye of Captain Eleanor Stack, before the ship sets out on a seven-month mission to the Mediterranean and Black Sea. As HMS Duncan passes into the Black Sea, the operation begins in earnest with the sudden emergence of a Russian warship that begins to follow it.

HMS Duncan is deep in the heart of the Black Sea as it heads towards Crimea, when a Russian ship it has been tracking suddenly disappears. Pilots Johnny Hamlyn and Tim Dunning take off in a Wildcat helicopter and discover that it is a spy ship, probably on a mission to monitor the Duncan. The situation soon escalates as Russian jets start circling the Royal Navy ship, and Commodore Mike Utley and Captain Eleanor Stack must assess the situation and consider how best to protect their personnel.

Reports that a chemical weapon has been used in Salisbury changes the mood onboard. It is alleged that the Russians are behind the attack and with the ship heading towards Montenegro the crew are on guard. When another chemical attack takes place, this time in Syria, the ship goes to action stations for the first time.

The crew have been sent to Ukraine, but the unexpected absence of Russian ships leaves everyone unsettled.

When Iranian forces take control of the British flagged tanker Stena Impero, Duncan is ordered to the region.

As Iran still holds the Stena Impero, Duncan is ordered to continue escorting fuel tankers safely through the Strait of Hormuz.

Travelling off the coast of Scotland, Northumberland receives intelligence about a Russian submarine that's currently under surveillance. New officer recruit Olivia joins for her first taste of life at sea.

HMS Northumberland is deployed off the west coast of Ireland and receives intelligence that a Russian warship has been spotted heading towards Faslane, the home of the British Trident submarine base. There is a concern that if gets too close it could pick up details of some the UK's nuclear secrets.

Preparing to leave once more, huge amounts of food are loaded along with the ship's new chef, but a fire nearly undoes the entire mission right as it's begun. Northumberland is also called into action to help when a Russian spy ship is spotted.