''Sweet Danger'' is the fifth book in Margery Allingham's ''Campion'' series, which has our eponymous gentleman-adventurer on a quest to find the rightful heir of a suddenly-valuable principality on the Adriatic Sea known as Averna. The British Government want proof of ownership and this, of course, involves overcoming several obstacles, including a curious riddle, collecting various items and keeping one step ahead of the enemy. The quest soon becomes a race against time, when the villains, led by Machiavellian schemer Brett Savanake, start to close in on our heroes.