 The film's storyline concerns a British Royal Navy artillery officer (Grant) who has orders to retrieve a huge siege cannon from Spain and transport it by ship to British forces.But first, the leader of the Spanish guerrillas (Sinatra) wants to transport the weapon across Spain, to help in the recapture of the city of Ávila from the occupying French before he releases it to the British.Most of the film deals with the hardships of transporting the big gun to Ávila across rivers and through mountains, while also evading the occupying French forces that have been ordered to find it.