A powerful alkaloid is the infernal plant extract used by the greedy brothers Ricardo and Martín De Rosales to control the bunch of crazy ruffians who call themselves the Locos, a gang of junkies who carry out armed robberies and terrorize the Rio Grande with their raids. The De Rosales find a hide-out in an inaccessible castle in the marshy Los Patos lagoon, and the Yaquis of the quack doctor Quemado – who are in league with them – busy themselves with picking and treating the ominous flower from which a diabolical hallucinogenic concoction can be obtained. The pards, after decimating the Locos at Bosque bonito, head off towards Los Patos together with El Morisco and Eusebio.