While the handling is fairly dull, the film's main plot develops into a sustained suspense situation as a British vessel, commandeered by stiff-upper-lipped John Mills, is planted with explosive charges by Italian naval officer Ettore Manni and his wounded companion, who are then imprisoned on the ship itself after refusing to give details of their mission including the whereabouts of the bomb itself.

An underwater search at night fails to reap the desired results and Mills with the help of officer Robert Shaw, who's married to an Italian girl,  determines to retrieve the necessary information which could save the ship and the life of more than a thousand men on it.

Doctor Liam Redmond opposes his treatment of the P.O.W.s, but remains on board to cure the injured man even after the vessel has been evacuated. The explosion eventually occurs early the next morning when the ship was scheduled to set sail for war duty; the film, then, ends with Mills awarding Manni for his integrity and loyalty to his cause three years after the fact.

As I said, the film is generally interesting though the interrogation/confrontation scenes do get repetitive; it's also bogged down by resistible comic relief provided by two marines appointed to guard the saboteurs.