Fifth in the Lieutenant St. Vincent Halfhyde series, this takes us once again back to the time when steam vessels replaced sailing ships in Queen Victoria's navy. Halfhyde and his little torpedo-boat Vendetta are off the coast of Gibraltar with the fleet commanded by blithering Captain Watkiss, an inflexibly idiotic disciplinarian, Halfhyde is given orders to go ashore on Torremolinos and meet secretly with the British diplomat Colonel Stanley. When they do meet, under cover of darkness, both are thrown into a Spanish dungeon where Stanley reveals that a Spanish nobleman has mounted a plot to assassinate Queen Victoria. Once they escape and are back with the fleet, they are no less safe, since the Spanish fleet has taken Watkiss' ships under arms and is searching them for Stanley. When Halfhyde and Stanley attempt another escape and fail, Halfhyde begins to suspect that Stanley is not all he pretends to be, so he takes the Vendetta and makes a run for it straight through the Spaniards. He must get to Scotland to foil the assassination scheduled to take place during the Queen's visit. The showdown comes in the Queen's railroad car over her breakfast as both Halfhyde and Stanley face her and pour out their stories. Derring-do, mystery and humor, but the age of steam is losing its novelty.