Louisa has a medical scare and Martin diagnoses officer Penhale's brother Sam with an illness.
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.