First of the Bulldog Drummond novels.
Bored former army officer, Capt. Hugh Drummond, “late of the Royal Loamshires”, puts an advertisement in the paper looking for adventure. He gets more than he expected when a young woman puts him on the trail of a master criminal who is organizing a would-be socialist putsch.
Entertaining in a square-jawed, stiff-upper-lip sort of way.