The police inspector Fonseca is interested in two Lisbon men in particular, the wealthy and notorious Greek criminal Aristides Mavros and an American suspected smuggler with a fast boat, Captain Evans.

Irish-born Sylvia Merrill, married to a much older millionaire Lloyd, is tired of waiting for government officials to rescue her husband, held as a prisoner behind the Iron Curtain.

[3] The topic of the film was sensitive under the Cold War perspective of the 1950s and there were initial concerns about how Paramount would deal with a story involving the Iron Curtain.

Luigi Luraschi, Paramount's Head of Foreign & Domestic Censorship at the time, wrote a report to the CIA in January 1953 stating that "[Lisbon] could be messy if mishandled, but so far we have them thinking along the right track and this could be very useful to us.

"[4] Irving Asher hired director Nicholas Ray and, in early 1953, Joan Crawford was in talks to star as main character Sylvia Merril.

Paramount then sold the rights to Republic Pictures and Herbert J. Yates hired Ray Milland for his second directorial effort and starring role in the film.

[7] Milland reportedly wanted A Man Alone co-star Mary Murphy for the female lead,[3] a part which ultimately went to Irish actress Maureen O'Hara.

Exterior shots included many locations in Lisbon, among them the Belém Tower, Praça do Comércio, the Castle of São Jorge and the Jerónimos Monastery.

Variety stated that "Lisbon makes a colorful setting" and "Republic's anamorphic Naturama process and Trucolor go a long way towards visual impressiveness.