Martin is shocked to find Louisa and Aunt Joan knocking at his door before his office has opened. Louisa has had a medical scare and wants Martin's reassurance that everything is fine. P.C. Penhale, still officially the world's worst policeman, has an unexpected visitor - his older brother Sam. Martin notices that Sam has some problems with his memory and co-ordination. It's probably just a virus, but when Martin takes Sam's family history, he decides there is a very small possibility that Sam has a rare genetic disease. It turns out that Martin diagnoses a different illness that will change Penhale's opinion of his brother forever.
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