Commodore Jack Aubrey, wearing his broad pennant in his favourite ship Surprise, has taken his squadron of frigates from Gibraltar down the North African coast to counter an attack by Moorish corsairs upon a fleet of East Indiamen.  The frigates race urgently across a moonlit sea, cleared for action, with battle lanterns lit along their gundecks and the glow showing here and there through open gunports.  The Hundred Days refers to the period in which the novel is set: Emperor Napoleon, escaping from exile on the isle of Elba, runs wild in Europe for a hundred days before meeting the Iron Duke, Wellington, at Waterloo.   
From The Marine Art of Geoff Hunt