This is the ninth in the Wallace Boys series. Following their adventure in London and Cornwall, the boys go to the far north-west of Scotland where they intend to refurbish a yacht, the ten-metre Silver Spray, prior to sailing her out to the South Pacific. They meet up with Richard Hannay, a boy of their own age, the grandson of the famous John Buchan character in 'The Thirty-nine Steps'.The Silver Spray is lying up on Loch Machray overlooked by a magnificent island castle similar to the famous, dramatic Eilean Donan, but no one can visit the island, for it is certain death for anyone who dares. Like Gruinard Island, this island too was infected with the deadly anthrax bacteria during a biological experiment during World War Two.Bruce then lands accidentally on the island in thick fog!And then the excitement really starts, involving the IRA and a royal hostage, secret tunnels, laird's lugs and a bottle pit dungeon!