When a mysterious beam of light starts disrupting and destroying the Earth's atmosphere, Flash Gordon (Larry "Buster" Crabbe), Dr. Zarkov (Frank Shannon), and Dale Arden (Jean Rogers) - accidentally accompanied by wisecracking reporter Happy Hapgood (Donald Kerr) - swing into action in Zarkov's rocketship, believing that it could be coming from the planet Mongo. Once in space, however, they discover that the ray is originating from Mars.
"The Octopus," a masked crime lord, is bent on crippling America with a wave of terror. He demands tribute from railroad magnates and other captains of industry. Richard Wentworth (Warren Hull), an amateur criminologist who is friendly with the police and is secretly "The Spider," a masked vigilante, is equally determined to destroy the Octopus and his gang. Pleasant and smiling in civilian life, Wentworth is frequently ruthless as The Spider, using his two .45 automatic pistols against any public enemies who attack him.
Lamont Cranston is an amateur criminologist and detective, who hosts a daily radio program sponsored by the Daily Classic newspaper. He has developed a friendly but occasionally terse feud with Police Commissioner Weston. Cranston complains to his managing editor, Edward Heath, about his incompetent new assistant, Phoebe Lane. Heath advises him that because she is the publisher's niece, she cannot be fired. During his radio broadcast, Phoebe gives Cranston a note that the Metropolitan Theatre is to be robbed at eight o'clock. Afterwards, he learns she got the information from a man she met in a café. Cranston goes to the theatre; Weston and his men have already arrived, but there is no crime. Across town, international banker Gerald Morton is killed and his safe is robbed.
In 1865, Captain Mark Smith of the Confederate Army leads a band of deserters to conquer Texas and rule it as a dictator. In one of his first actions, he captures and assumes the identity of Texas' new Finance Commissioner, Colonel Marcus Jeffries, after having the real man murdered. When a contingent of Texas Rangers enters the territory, Snead, one of Smith's men, leads them into an ambush by Smith's "troopers". The Rangers are apparently wiped out, although one injured survivor is left. The survivor, nursed back to health by Tonto, swears to avenge the massacre and defeat "Colonel Jeffries" and his men.
Lamont Cranston assumes his secret identity as "The Shadow", to break up an attempted robbery at an attorney's office. When the police search the scene, Cranston must assume the identity of the attorney. Before he can leave, a phone call summons the attorney to the home of Delthern, a wealthy client, who wants a new will drawn up. As Cranston meets with him, Delthern is suddenly shot, and Cranston is quickly caught up in a new mystery.
In contemporary (for the 1937 production) California, villain J. A. Marsden aims to take over the California-Yucatan Railroad with the aid of his henchman El Lobo. The rightful owners, Joyce and Phillip Andrews, naturally object. Their parter, Don Manuel Vega summons his nephew, James Vega, to help them as he is the great grandson of the original Zorro, Don Diego de la Vega. He is disappointed, however, to find that his nephew is a useless fop (presumably Don Manuel had not paid too much attention to his family history).
Zorro (Robert Livingston) has been captured and set for execution, charged with the murder of the new Governor (Robert Warwick) in Spanish California, as the Governor was marked with a "Z". Zorro escapes, and reveals his identity to the Governor's daughter, Isabella (Heather Angel). However, Isabella then has Don Diego arrested. He convinces Isabella that the Commandante (Sig Ruman) was the real killer, as the "Z" on the Governor was backwards. Eventually, Isabella helps free Don Diego, the Commandante is killed, and Diego and Isabella reunite.
The Planet of Peril
The planet Mongo is on a collision course with Earth. Dr. Alexis Zarkov takes off in a rocket ship to Mongo, with Flash Gordon and Dale Arden as his assistants. They find that the planet is ruled by the cruel Emperor Ming, who lusts after Dale and sends Flash to fight in the arena. Ming's daughter, Princess Aura, tries to spare Flash's life.
Don Diego de la Vega (Zorro)'s son, Cesar (Douglas Fairbanks), is in Spain finishing his education. While Cesar is showing off to friends his remarkable prowess with the whip, he accidentally clips off the feather shako on the hat of Don Sebastian (Donald Crisp) of the Palace Guard. Although Cesar apologizes immediately, Sebastian is unforgiving. Their duel is interrupted by a runaway bull. Trapped on the ground with his sword belt tangled in his boot, certain to be gored by the bull, Sebastian is saved at the last minute by Cesar. This further infuriates him. The action is observed by Queen Isabella (Stella De Lanti) and her guest, Austrian Archduke Paul (Warner Oland); she requests Cesar's company immediately. Another friend of Cesar, Don Fabrique Borusta (Jean Hersholt), offers to bring him to Her Majesty.
The Mark of Zorro tells the story of Don Diego Vega, the outwardly foppish son of a wealthy ranchero Don Alejandro in the old Spanish California of the early 19th century. Seeing the mistreatment of the peons by rich landowners and the oppressive colonial government, Don Diego, who is not as effete as he pretends, has taken the identity of the masked Robin Hood-like rogue Señor Zorro ("Mr. Fox"), champion of the people, who appears out of nowhere to protect them from the corrupt administration of Governor Alvarado, his henchman the villainous Captain Juan Ramon and the brutish Sergeant Pedro Gonzales (Noah Beery, Wallace Beery's older half-brother). With his sword flashing and an athletic sense of humor, Zorro scars the faces of evildoers with his mark, "Z".