Bob and Barbara, Jack and Caroline. Two happily married couples who have been great friends for so long. But all is not what it seems. For Barbara does not know that when Bob stays overnight with Jack and Caroline, Jack is never there. But, one night, arriving early for his usual intimate dinner with Caroline, Bob find murder instead of passion at his friends' house. Desperately he tries to get hold of Jack to make it look as if his presence there is as innocent as it is supposed to be. Detective Chief Inspector Hewitt is sure that both Jack and Bob are concealing information from him. But it takes a second murder attempt and an anonymous letter before he is able to discover the terrible truth.