The Island of Sheep is a novel by John Buchan. It is part of the series featuring Richard Hannay

Richard Hannay

Major-General Sir Richard Hannay, KCB, OBE, DSO, Legion of Honour, is a fictional secret agent created by Scottish novelist John Buchan. In his autobiography, Memory Hold-the-Door, Buchan suggests that the character is based, in part, on Edmund Ironside, from Edinburgh, a spy during the Second Boer...

 and Sandy Arbuthnot.
The action occurs some twelve years later on from the last novel, when Hannay, now in his fifties, is called by an old oath to protect the son of a man he once knew, who is also heir to the secret of a great treasure. He obtains help from Sandy Arbuthnot, now Lord Clanroyden, and Lombard.