A highly valuable necklace is stolen from Lord Gillingham by his secretary Kate, who kills the jeweller in the process. The necklace is discovered at a fashion parade... or is it? The model sporting it is actually a twin and it would seem that her sister has decamped to Paris to sell the real thing to a wealthy dealer. Or has she? A double switch seems to be on the cards when his Lordship gets a telephone call saying that he can have the necklace back if he pays ten thousand pounds. What would anybody do in such a predicament but call for the Saint?