Halfhyde for the Queen — Philip McCutchan

In Andalusia, Lieutenant St. Vincent Halfhyde is sent by the unpredictable Captain Watkiss on a secret mission ashore: to aid the Queen's Messenger, who is being pursued by nefarious Spanish agents. But when Halfhyde discovers a plot to kill Queen Victoria herself, the stakes are raised. Now he and the rest of his flotilla must face down the Spanish Navy and get the Messenger back to Britain with vital information.