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.

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 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.