A murderer is brought to court and only Miss Marple is unconvinced of his innocence. Once again she begins her own investigation.
John (Alan Ladd), a blacksmith and swordsmith, is tutored at Camelot. As a commoner, he can't hope to win the hand of Lady Linet (Patricia Medina), daughter of the Earl of Yeoniland (Harry Andrews), so he creates a secret alternate identity as the Black Knight. In this new role, he is now able to help King Arthur when Saracens and Cornish men—disguised as Vikings -- plot to take over the country.