While hunting German shipping, the Royal Navy submarine Trojan accidentally strikes an electronically-operated mine - and the race begins to save the lives of the crew of the stricken vessel. Rocked by a tremendous explosion, the Trojan plunges to the sea bed. As time - and air - starts to run out, the Captain gathers together the survivors. A lucky few can escape immediately through emergency hatches - but the others will have to remain trapped in the dying submarine and hope that rescuers will arrive before it is too late. Those left behind must battle with terrifying fear and claustrophobia in the stricken, disintegrating hulk far beneath the sea while rescuers begin a last-ditch attempt to raise the submarine to the surface. The rescue is difficult and fraught with danger at the best of times - but a storm of unprecedented ferocity is brewing which threatens to end all hope of rescue.