This is quite an anxious wartime drama that goes some way to illustrating the perils faced by those in HM Submarine Service. John Mills is the captain "Armstrong" who takes his crew on a routine maritime patrol only for their gear to become ensnared in a mine cable. A suddens stop and reverse engines doesn't quite do the trick and to the bottom they go. Largely in one piece and not in such deep water, they are optimistic of rescue and, indeed, help arrives fairly sharpish allowing the first four of the twelve survivors to don their emergency gear and head to the surface. The sense of enclosure now builds as their air starts to thin, the first officer "Manson" (Nigel Patrick) becomes ill and we discover that "Snipe" (Richard Attenborough) only joined the service for the extra pay and he actually does suffer from claustrophobia. With four gone, its now the turn of the next four - but there is a snag. The damage from the explosion has ensured that these are the last four escape kits. A lottery needs to be held and that further raises the tension as those left behind will have to await the raising of the ship - and that's dependent on fair weather above! The story develops well here, with Mills (who did like snapping his fingers a lot in these roles!) working well with Patrick, an on form James Hayter as the cook and general dogsbody "Higgins" and Attenborough who really does present us with a plausible sensation of his panic at being shut up in this metal tube deep under the sea as well as the growing guilt he feels at the selfishness of his behaviour. The script marries a bit of dark humour with the accruing peril and Roy Baker keeps the pace taut for the duration of this rather more impactful story.