Travelling alone from India to England in 1940, newly married Cissie Humphreys is captured after her ship is torpedoed. Facing the stark choice of either prison or collaboration with the enemy, she works for their propaganda service and falls in love with charismatic Sean Munroe, an IRA rebel. After training as a secret agent, Cissie accompanies Sean on a spying mission to a small fishing village on the north Yorkshire coast. But it is not long before Cissie alone must face the consequences. Now the only chance to save herself is to act as a double agent for the British. Where will it all lead?