In order to learn the location of a fabled Aztec treasure, a professor kidnaps his colleague, the only man able to read the ancient Aztec script that is supposed to reveal the location of the treasure. Charlie Chan and his #1 and #2 sons journey to the jungles of Mexico to find the victim and bring the kidnapper and his gang to justice.

Sandy Doyle, gambler and political chief of a small border town, seeks to gain control of the Bar-X Ranch, owned by Rufe Rickson, to further some undercover activities of his own. He counts on Rickson's inability to stay away from gambling as the means to his ultimate success. Government investigator Oliver Shea and his assistant, Dan Haggerty, start a fight in Doyle's place when they see Rickson being cheated and are invited to the Bar-X where Oliver and Helen Rickson, Rufe's daughter, discover interest in each other and Dan finds himself pursued by Bell, the ranch cook. Sheriff Larson brings the prize money for the $5,000 race of the Rodeo Association, and that night it is stolen.

In occupied France, a convicted thief and murderer escapes the guillotine when a bombing raid strikes the prison, but is quickly re-captured by the inspector of the Surete responsible for his original arrest. Fearing the guillotine more than his actual death, the convict inveigles the inspector to help him with a plan to rescue 100 Frenchmen taken by the Gestapo following an act of sabotage: he will confess to being the saboteur and allow himself to be executed by firing squad, the Gestapo's method of execution, thus freeing the 100 men.

A scientist working on cures for rare afflictions, such as a bone softening agent made from molds to allow him to correct the spinal deformity of his nurse, finds the physical causes of lycanthropy in wolf-man Larry Talbot and of vampirism in Count Dracula, but himself becomes afflicted with homicidal madness while exchanging blood with Dracula.

Both Harmon and his men and a Mexican gang are after a treasure hidden on the Wilson ranch. Acey learns of their raid and goes to get Ken only to find him in jail for a murder he did not commit.

Magazine photographer Ann Larkin is snapping photos at Mexico's National Museum when she sees Brod Williams steal a painting from its frame. Convinced that Brod is the notorious art thief known as "The Wildcat," Ann follows him into the street and accuses him of being the thief. Even though the police attest that Brod is a New York City police detective, Ann remains dubious.

Abbot and Costello must find a replacement for a woman's horse they accidentally killed after feeding it some candy. They head for the racetrack, find a look-a-like and take it. They do not realize that the nag is "Tea Biscuit," a champion racehorse.