Humans live in the world of Assiah, demons in Gehenna. The two dimensions are not meant to interfere with each other, but demons still possess creatures in Assiah in spite of this. The humans who can fight these demons are known as exorcists. Rin Okumura is a boy who bears the curse of being Satan's illegitimate son. His foster father sacrificed himself to save him from demons. To avenge his foster father's death as well as to prove himself, Rin decides to follow the path of an exorcist and defeat his own father, Satan. To hone his raw skills, Rin enters True Cross Academy to train with other exorcist candidates.

Much more than just the world's largest Cross: the Valley of the Fallen [Valle de los Caídos] is history carved in stone. It's the Basilica, the abbey, the College-Choir School, the guesthouse. Besides being a place of worship, since 1958 it has been home to about twenty Benedictine monks. Some of the first ones still live there. It also housed, until 1982, the Centro de Estudios Sociales [Center for Social Studies]. All of this is now under threat.

This documentary aims to shed light on the history and significance of the Valley of the Fallen, explaining when, how, and why it was built, while debunking the main myths surrounding it.