HMS Trojan charts a tense, somber ordeal after a routine snorkel-mast test goes catastrophically wrong. The British submarine plunges into calm waters, only to confront an unrecovered Second World War magnetic mine. When the mine detonates, the bow is ripped away and the after section floods, killing 53 crewmen and leaving twelve wounded and stranded amidships. The vessel settles on the sea floor, and a handful of sailors—miraculously saved by watertight doors that held—live on.
The shore base grows anxious as Trojan remains overdue. Rescue ships sweep the area, and the captain uses a dramatic signal—a plume of oil rising to the surface—to pinpoint the submarine’s position. A diver descends with an air line, and the crew prepares for a perilous salvage operation. The initial rescue goes ahead: the first four men are released and hauled to the surface through the gun hatch. Among the survivors below, the remaining eight assume there are enough breathing apparatuses to free everyone, but the captain soon discovers that most sets were destroyed in the blast.
Armstrong, the ship’s calm and methodical commander, tries to manage the crisis with a stark, almost clinical approach. He gathers the crew around a deck of cards and uses luck to decide who will be airlifted first and who must stay behind. The cook, A/B Higgins, and Lt. Manson draw low cards, while three others draw high cards. There is a tense tie between Stoker Snipe [Richard Attenborough] and E.R.A. Marks [George Cole]. After a re-deal, Snipe’s fear erupts, and the young man must be restrained. Armstrong then asks Marks to stay, and Marks agrees to remain with the men below.
The four chosen for escape move through the conning tower and are picked up by the surface salvage team. Below deck, Lt. Manson experiences a fainting spell, which he attributes to prior malaria—though Snipe notices he can lift Manson with both arms without difficulty. The mood above and below is initially hopeful, bolstered by steady weather as divers secure the necessary cables and Trojan is cautiously winched upward.
Yet the weather turns vicious. A storm rips across the sea, and the submarine shifts on the cables, sinking again to the ocean floor. The fragile balance below is worsened when chlorine leaks from a nearby site, and Manson is overcome by the gas and dies. The salvage crew, recognizing the danger to their own lives, finally abandons the operation, leaving the three men still aboard with a sense that all hope is slipping away.
In the end, the rescue mission grinds to a halt, the weather worsens, and the three who remain on Trojan confront a grim, solitary fate. Armstrong’s final moment is quiet and reflective as he reads from a naval prayer book, a stark contrast to the ship’s earlier, more practical measures.
Across the film, we glimpse intimate facets of the crew’s lives. Stoker Snipe is shown as a man whose sense of duty wrestles with a personal vulnerability, his loyalty tested by fear. Armstrong himself is revealed to be considering a shore posting offered by a wealthy father-in-law, a plan that would pull him away from the Navy once this patrol ends. These personal strands deepen the procedural drama, anchoring the submarine’s crisis in human stakes and quiet, unresolved longing.
