Fiftieth in the long-running mystery series with Dr Launcelot Priestley.

In the Norfolk Broads one of identical twin brothers drowns in a boating trip tragedy. From the second the survivor plans to step quietly into the shoes (and wealth) of his unfortunate sibling, events take a decidedly dangerous turn.

It emerges that far from being the respectable citizen everyone thought, his brother was involved in some extremely dubious enterprises. The survivor attempts to hide himself away at a rest home where his brother had stayed the previous year - but this proves unsuccessful in shielding him from danger.

The suspicions of the doctor who runs the home give Superintendent Waghorn the first clue he needs to begin to unravel the case – as well as providing the `two graphs' of the title – and with Priestley's help he manages to track the criminal.