A Swedish atomic scientist, Professor Hansen, is kidnapped in Stockholm by Nazi agents and brought to the neutral Republic of Ireland to continue his work. The Nazis hope to build an atomic weapon. British Intelligence gets wind of this and sends two undercover military intelligence agents to infiltrate the Nazi spy network and find the kidnapped scientist. The British agents are Capt. David Grant and Capt. Toby Hunter. The opposition is comprised of Paul Faber, network chief and various underlings such as Keitel, Bowman, Lily and George Leggett and phony Austrian refugee Marion Decker. British Police Inspector Emerson is assisting British military intelligence and Sir George Bell is the British Government representative assigned to the case. The Nazi spy network operates in both London and Dublin. The Nazi agents frequently travel from the UK to Ireland by ferry boat, hence the title of the film.