Fiero is hired to assassinate the newly-elected civilian president of San Pedro. His employers are a group of generals, threatened by the liberal sympathies of the new president-elect. The generals, having killed Fiero's father years ago, trick Fiero into thinking he is working for the leftist People's Party. Meanwhile, a CIA agent is sent stop to the assassination. It's a race against time as the agent must stop the HOUR OF THE ASSASSIN.
Juan Sin Fiedo (Javier Solis), Juan Colorado (Antonio Aguilar), Juan Pistolas (Luis Aguilar) and Juan Charrasqueado join their destinies during the Mexican Revolution.
Young doctor Jaime attends his father's clinic where Diana, a singer, is carried by emergency. After operating it, Jaime falls in love Diana and decides to accompany her throw artistic caravan, where replaced a singer and quarrels by jealousy with Javier, a womanizer trying to conquer Diana.
After a young man is killed in an unexplored subterranean cavern and his girlfriend driven mad by something she saw there, a track found at the scene cannot be identified with any known animal and thus a scientific expedition is launched to find out just what it is that is living there.