The Unknown Quantity by M. G. Eberhart

This is the story of an utterly vile man who is caught out doing something nowhere near vile enough to suit his character. Successful businessman Arthur Travers tells his young wife Sarah that he's been called away on urgent secret government business, and suggests that he should be impersonated in his absence by a lawyer, Dixon, with whom he shares an uncanny resemblance. This ruse, he explains, will throw enemy agents off his trail, as they will believe he is at home rather than abroad on an official mission. Little does Sarah know that Arthur's real purpose is murder, and Dixon's presence at her home is to be his alibi. When a body is found in the park where Arthur failed to keep his rendezvous with Sarah, she and Dixon are left to face some fearful choices.

It was around halfway through the book when I worked out that Arthur was not only still alive, but also genuinely up to no good. From then on the narrative dragged a bit, as it was all focused on Sarah and Dixon's increasingly frantic attempts to prove that they were not responsible for Arthur's supposed death. Through this adversity they inevitably develop an unbreakable bond, but that's all right, because we're told frequently that Arthur never loved Sarah anyway.

At length there is a very complicated climatic scene involving a boathouse at night, an inevitable case of mistaken identity between Arthur and Dixon, some inept rowing by Sarah and the sudden arrival of apparently hundreds of State Troopers. By the time we've fought our way through this tangled mess of plot and setting, Arthur is dead and all has been explained as an ancient dispute with an embezzler whose ill-gotten cash Arthur "looked after" a little too well.

Explained, yes. Satisfactorily, no. It was all far too complex for the malnourished plot it was supporting. This story could have been told in half the space, and everyone would have been better off for it.