A champion jockey is banned from racing so spends his time helping a young lad to become the next champion.

The San Demetrio of the title is a British merchant ship in an Atlantic convoy in 1940. Disabled and left to the mercy of patrolling U-boats the crew must keep her afloat and out of harms way.

An insane murderer is on the loose, and gunning for the men who put him away. Will Hay is on the list, and co-opts Claude Hulbert to try and stop him from meeting a grisly end. This black comedy suggests that, had he lived, Will Hay might have moved on to the darker corners of Ealing comedies such as 'The Ladykillers'.

Francis Barnard goes to Spain, when he hears his sister Elizabeth has died. Her husband Nicholas Medina, the son of the brutest torturer of the Spanish Inquisition, tells him she has died of a blood disease, but Francis finds this hard to believe. After some investigating he finds out that it was extreme fear that was fatal to his sister and that she may have been buried alive! Strange things then start to happen in the Medina castle.