When Secret Service agent David Somers is fired, he takes a quiet job with the Fentons at their country estate - cataloging butterflies, hence the title insect. David grows fond of Jess Fenton's niece, a fragile, fey young woman named Sophie. Because he hates traps of any kind, he reacts quickly when Sophie is framed for the murder of Hick, the nasty handyman. He helps her escape London by using his agent's skills and a network of old friends. The pair lead the police and David's ex-employers an exciting chase, from Newcastle to the Lake District to Liverpool. As the fugitives try to catch a ship for France, everyone, including the murderer, join in the finale.