Swordsman of Siena (Italian: La congiura dei dieci; French: Le Mercenaire) is a 1962 French-Italian adventure film directed by Étienne Périer and Baccio Bandini and starring Stewart Granger, Sylva Koscina and Christine Kaufmann.

[4] Thomas Stanswood, a veteran English mercenary serving the nobles of Renaissance Italy, accepts employment in the city-state of Siena.

This former republic has been occupied by the Spanish Empire, and Don Carlos, the cold-blooded Imperial governor, seeks to legitimize his power by marrying into the locally prestigious Arconti family.

Stanswood must protect the bride-to-be, Lady Orietta Arconti, whose friendly relations with the Spanish have put her in danger of assassination by a mysterious resistance group known as "the Ten".

Attending Don Carlos' court the next morning, the mercenary sees that Andrea Paresi, a Sienese nobleman, bears an identical wound on his hand.

Ever since the Spanish invasion this event has been won by Hugo, captain of the Imperial guards; victory by an Italian would cause a surge of local pride that the Ten could exploit to overthrow Don Carlos' government.

In conversation with Stanswood, Serenella learns that his native town in England is also oppressed by a tyrannical government, and that he plans someday to liberate it with the help of the money he has made in Italy.

The guards captain is unhorsed and killed by an angry mob, while Stanswood rides into the city and incites the Italians to rise up and capture Don Carlos.

Stanswood also tries to leave, intending to resume his mercenary career, but Orietta sends a party of armed men to bring him back.

[8] In November 1954 the studio said the film would be made under the direction of Andrew Marton, and that Granger's co star would be Gina Lollobrigida making her Hollywood debut.