In 1947, the long-retired Sherlock Holmes, aged 93, lives in a rural Sussex farmhouse with his widowed housekeeper Mrs Munro and her young son Roger. Having just returned from a trip to Hiroshima, Holmes starts to use a prickly ash plant he acquired there to try to improve his failing memory.

Unhappy about Watson's fictionalisation of his last case, The Adventure of the Dove Grey Glove, he hopes to write his own account, but has trouble recalling the events. As Holmes spends time with Roger, showing him how to take care of the bees in the farmhouse's apiary, he comes to appreciate Roger's curiosity and intelligence and develops a paternal liking for him.