The narrator recounts a campaign of violent outrages carried out under the signature Captain Shannon, including a devastating explosion at police headquarters, and undertakes a personal investigation. Tracing clues across England and Ireland, he pursues a slippery suspect known as James Mullen (also Henry Jeanes), examines crucial documents, assumes disguises, and infiltrates ships and coastal quarters. The plot interweaves detective work, conspiratorial maneuvering, narrow escapes, and explosive devices as the narrator pieces together identity and motive, ultimately confronting the network behind the terror and bringing the principal offender to arrest.