Erstwhile detectives, Edwin and Charles, have helped their 'mentor' the Marquez de Marquis capture an immortal killer. But after more snooping, the boys begin to suspect that they've caught the wrong man, and their mysterious benefactor may not be what he appears. To make matters worse, it looks like the usually helpful Mad Hettie has joined the forces of evil as well. Can the boys uncover the truth, save the day and get the girl, all while maintaining proper British manners?