On his way to visit friends at Waddam Hall, Richard Hannay encounters Lady Madrigal Fitzjames, an impoverished member of the aristocracy, who is sharing the same railway compartment. By coincidence, Lady Madrigal is also on her way to Waddam Hall.

Due to muddled directions, both Hannay and Lady Madrigal end up at Waddam Manor, not Waddam Hall; here they are mistaken for the newly married couple Sir Gerald and Lady Masefield.

A fabulously valuable diamond necklace known as 'The Masefield Diamonds' plays a large part in the rest of the proceedings, which involves an international gang of jewel thieves, servants in the attic, abductions, mistaken identities, and swashbuckling adventure.