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.

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.

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

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.

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.