St. Vincent Halfhyde, recently separated from Her Majesty's Royal Navy, finds himself once again in danger when his attempts to establish a shipping line lead him into a confrontation with a company of ruthless, desperate men. After Halfhyde becomes the new master and owner of the Taronga Park, the first commercial vessel in the Halfhyde Line, he takes the elderly, only-just-seaworthy steamship and her crew on a maiden voyage from Sydney, Australia, to Queenstown in the south of Ireland. But in the Victorian Era, when coal-burning tramp steamers had only just replaced sleek clipper ships, a voyage from Australia to Ireland would have been far from routine. Severe storms and other dangers commonplace as pre–Panama Canal sailors rounded Cape Horn.

If Halfhyde thought that leaving the Navy meant leading a quiet life, he was wrong. While in the Pacific, his ship is taken over by the man from whom he obtained his cargo: Porteous Higgins. Unfortunately, a number of Halfhyde's recently-signed crew are also employed by Higgins. And the ship’s cargo, said to be cased machine parts, turns out to be guns and ammunition destined for rebel enclaves in the west of Ireland. After attempting to warn the British Government, Halfhyde completes the voyage under Higgins’ command. When the British Naval authorities fail to react, Higgins and his thugs are able to enter Cork Harbour and leave the ship with the cargo. But Halfhyde and loyal members of his crew head north, determined to thwart Porteous’ schemes, and eventually confront the rebels themselves in the wild, mountainous territory of Connemara.