"Dave Darrin on Mediterranean Service; or, With Dan Dalzell on European Duty" by H. Irving Hancock is a naval adventure novel written in the early 20th century. The story follows two young ensigns, Dave Darrin and Dan Dalzell, as they encounter mystery and danger while serving aboard the battleship "Hudson" in the Mediterranean during World War I. The plot kicks off with the two friends in Gibraltar, where they get tangled up in a robbery involving a secret government document, setting the stage for further thrilling escapades. The opening portion introduces Darrin and Dalzell during an evening out at a theatre in Gibraltar, where they witness a brawl instigated by a suspicious character known as Mr. Green Hat. This leads to their involvement in the pursuit of a thief who steals a significant document from an American secret service agent, George Cushing. As Darrin and Dalzell navigate the ensuing chaos, they are swept into a deeper plot involving espionage that threatens to bring the United States into conflict with Great Britain. The narrative sets a gripping tone filled with camaraderie, suspense, and the perils of international intrigue, hinting at the larger challenges Darrin and Dalzell will face throughout their Mediterranean service. (This is an automatically generated summary.)