In Southampton in 1916, HMHS Britannic, a sister ship of the Titanic, has been refitted as a hospital ship for Allied soldiers fighting in the Gallipoli Campaign. Among the nurses who are to serve aboard her is Lady Lewis, who is being delivered to Greece via Naples, where her husband has become Ambassador for Great Britain. Traveling with her is Vera Campbell, an operative of British Intelligence posing as Lady Lewis' governess. She is unnerved by the voyage, having survived the Titanic sinking four years before, losing her husband in it as well. She reports her mission to Captain Bartlett who is dubious that a woman can do such a job. A German spy has boarded the Britannic posing as her chaplain, Chaplain Reynolds, and soon discovers that she is secretly carrying a large amount of small arms and munitions bound for Cairo. Under the articles of war, Reynolds considers his actions against her to be legal and initiates a series of sabotage attempts to either take over or sink her, including inciting the Irish stokers, all members of the Irish Republican Brotherhood, to mutiny. Each sabotage attempt is foiled by Campbell - with the eventual co-operation of the Britannic's crew. Unaware that she is responsible, Reynolds finds himself growing attracted to her whilst the voyage continues. As they spend time together, they fall in love and she has sex with him before discovering his true identity. Campbell confronts him in the engine room where he tells her he is going to sink the ship. Reynolds blows a hole in the Britannicport side bow. The ship tries sailing for Kea island seven miles away but the beaching operation causes her to sink even faster. Campbell discovers that William, one of Lady Lewis's children, has disappeared. Reynolds helps her and they manage to get William to a lifeboat before it is lowered. Another massive explosion causes Reynolds to be trapped in a flooding room. Campbell helps him escape and they make their way through the ship, swimming through flooded rooms, vents, grates, and corridors, eventually making it outside by swimming through a porthole and climbing aboard an empty lifeboat that was already lowered into the water, but still attached by its ropes to the davits. Campbell and Reynolds notice a lifeboat filled with people getting pulled into the still spinning propellers. They watch in horror as the boat and its occupants are smashed to pieces by the spinning blades. Reynolds ties Campbell to a line thrown to them from a nearby lifeboat. Despite her protests that they both can be pulled to safety, he throws her into the sea after kissing her. Soon after, the lifeboat's ropes break and it begins to also get sucked into the propellers. Reynolds decides to commit suicide, staying aboard the lifeboat as it is smashed by the blades. A few moments later, the Britannic rapidly rolls over, causing her funnels and deck machinery to tumble into the sea as she sinks beneath the waves. A British dreadnought, HMS Victoria, arrives to rescue the survivors. Reflecting on her experience, Campbell quotes the poem "Roll on, Thou..." from Childe Harold's Pilgrimage by Lord Byron.
