I will start with an anecdote (which is real):
In order to talk about this sucessful adaptation from one of the best corridos I ever heard, we must talk first about the man behind the creation of it's song; we must talk about mr. Paulino Vargas(R. I. P). Paulino Vargas Jimenez born in Promontorio, Santa Maria del Oro, Durango, Mexico in 1939. At the age of 13, Vargas met Javier Nuñez, with whom he later will form ''Los Broncos de Reynosa".
Years later, Vargas decided to learn to read and write (according from an online spanish interview published online); and once he had accomplished it at the age of 22, he decided to read plenty of stuff, including history. And thanks to that, Vargas began to cultivated literature ideas to create some of the hundreds of songs he composed; and here's an list of some of he's compositions:
1: La Banda del Carro Rojo.
2: El Corrido de Lamberto Quintero.
3: Carga Ladeada.
4: La Fuga del Rojo.
5: El Corrido de Chico Fuentes.
6: R-1.
Etc,etc.
Unfortunately, Paulino Vargas died at the age of 70 on January 17, 2010.
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Now is time to talk about this successful adaptation:
Even if this movie is a work of fiction, based on the song of the same name (Clave 7); the story behind the song is based on the death of Pedro Aviles Perez, a powerful drug dealer, and the event took place on September 15, 1978; when Aviles and a group of he's people where heading to a certain location in Sinaloa. And they were shot on a highway on as ''La Y", by the army. (Later this event will be immortalize by Paulino Vargas' Clave 7 song; which tell how Aviles' death took place, and it had been performed by plenty of music bands.)
Even if the movie's story has plenty of fiction (because it is very well understood, that ''they" have 1 hour to tell the story) the movie it is completely loyal to the details told by the song; and also for very obvious reasons that we know, some names, and some situations had been changed.
My final opinion: La Clave 7 is another one of few movie adaptations, that manage to stay loyal to it's song (and like I had said on other reviews: it also have to do with who wrote/adapt it, which in this case two famous celebrities contributed for the success of it: Gilberto de Anda (wrote the "Story by"), and Jorge Reynoso (Directed and wrote the ''Screenplay by").