An experimental vaccine is stolen, and the subsequent test proves deadly. Steed and Keel, suspecting that the human 'guinea pig' did not die directly from the vaccine, volunteer to be the next subjects.

Several notorious criminals have vanished into thin air. A whisper has been received that the escape chain starts in London, but, when agent Paxton follows it, he winds up dead in the Thames with a 300-year-old bullet in him.