Life on board nuclear submarine HMS Trenchant on its four-month tour of duty to the mid-Atlantic, uncovering the reality of life at sea. Her crew prepare the submarine to leave port for the first time in six months, with enough food coming on board to last for up to 120 days underwater. Their mission is to travel to the mid-Atlantic to protect a British submarine that's carrying Trident nuclear missiles.

Captain Stephen Brian takes command of HMS Trenchant and his arrival coincides with a growing threat from Russia, as their warships and submarines make forays into British waters. As everyone preparess for the tough realities of life back at sea on board a submarine, orders are received for Trenchant to stop the next Russian vessel making its way towards British waters.