Young Somerville, at school in London, makes off with the idea of watching the crowds who throng the ice on the Thames in January 1740. Boy-like, he lingers, and darkness has fallen when he is still far from the academy in Red Lion Square.
Suddenly a man appears ahead; it seems to be Somerville senior, and when he is attacked by footpads, the youngster comes to the rescue. Hours later he awakes in a strange room to find that he has been knocked out in helping Captain George Anson, of the Royal Navy.
Explanations and suggestions follow – Somerville sails with Anson in the famous Centurion as a midshipman.
Thereafter his life is part of Anson's astonishing voyage round the world, though a mysterious talisman, already responsible for much, brings adventures and dangers which concern Somerville alone. It is after a stirring naval action that Somerville sees the "villain" of the piece in tragic circumstances for the last time. He recovers the talisman, but when he reaches home after nearly four years of peril and not a little hardship, he gives the strange stone – the talisman – little credit for bringing him happily through.