Top Secret is the fifth and final title in what is known as the Chudleigh Hold series, only connected because its hero, Hawk, is a Chudleigh.
Owing to the murder of the original messenger, Hawk Chudleigh, going to work in Queensland, is the bearer of papers about a new secret weapon. Leakages at headquarters put certain men on the trail and his life is attempted during the voyage. Before anything can be accomplished, the steamer is blown up by a wandering mine, and Hawk and his chum, Walter Urquhart, escape by almost a miracle. They reach an island leased by Philip Penrose, a New Zealander, and make friends with him, his sister and other members of the Penrose family, who rally round them when the enemy raid the island.