Question: is the movie the mission based on a true story

Answer: Yes, The Mission is based on events surrounding the Treaty of Madrid in 1750, in which Spain ceded part of Jesuit Paraguay to Portugal. A significant subtext is the impending suppression of the Jesuits, of which Father Gabriel is warned by the film's narrator, Cardinal Altamirano, who was once himself a Jesuit. Altamirano, speaking in hindsight in 1758, corresponds to the actual Andalusian Jesuit Father Luis Altamirano, who was sent by Jesuit Superior General Ignacio Visconti to Paraguay in 1752 to transfer territory from Spain to Portugal. He oversaw the transfer of seven missions south and east of the Río Uruguay, that had been settled by Guaraní and Jesuits in the 17th century. As compensation, Spain promised each mission 4,000 pesos, or fewer than 1 peso for each of the circa 30,000 Guaraní of the seven missions, while the cultivated lands, livestock, and buildings were estimated to be worth 7--16 million pesos. The film's climax is the Guaraní War of 1754--1756, during which historical Guaraní defended their homes against Spanish-Portuguese forces implementing the Treaty of Madrid. For the film, a re-creation was made of one of the seven missions, São Miguel das Missões.