Steve Costigan and his companion Johnny Plunkett are en route from Quito to Valparaiso in a decrepit airplane when they are forced to make an emergency landing in Puerto Grenada due to mechanical failure. Their crash disrupts a bullfight by killing the main attraction—a prized bull—much to the fury of the locals and the dictator, Don Rafael Fernandez Pizarro. To atone for the bull, Costigan is coerced into replacing it in a public spectacle by fighting the famous bullfighter-turned-boxer, Diego Zorilla, who is also known as Diego the Lion.

As the event is prepared, the atmosphere in Puerto Grenada is festive yet tense, with Costigan under close watch by the local military. Meanwhile, Captain Richard Stark, another American caught in a financial dispute with Don Rafael, plans a daring escape for his crew and potentially for Costigan and Johnny as well.

The boxing match begins under the hot Puerto Grenadan sun, with the crowd initially hostile to Costigan. Diego employs a predictable strategy, relying on a powerful but telegraphed right hook, which Costigan easily counters. Despite the heat and the crowd's antagonism, Costigan manages to dominate the early stages of the fight, drawing blood and knocking Diego down.

However, just as victory seems assured, Costigan unwittingly drinks "Grenada Lightning," a local liquor, offered by a seemingly friendly spectator. This potent drink dramatically affects him, causing disorientation and impaired vision—suddenly, he sees multiple versions of Diego. Despite the alcohol's effects, Costigan's resilience shines through. He endures, powered by the potent drink, and manages a knockout blow when he correctly guesses the real Diego among his tripled vision.

Following the knockout, as the crowd and Don Rafael reel from the upset, Captain Stark's crew successfully uses the commotion to break out of jail. They converge on the plaza, aiding Costigan and Johnny's escape amidst chaos. As they flee to Stark's waiting ship, Don Rafael's control unravels, symbolized by his ignominious fall from power when Costigan, in a final act of defiance, physically assaults him.

As they sail away from Puerto Grenada, Costigan, Johnny, and Stark leave behind a town in disarray but carry with them tales of an adventure that epitomizes the unpredictable, rough-and-tumble life of Steve Costigan.