The Purple Monster Strikes is a 1945 Republic Movie serial. It was also released as a Century 66 television film under the title D-Day on Mars (1966).

The original production title for the serial was The Purple Shadow Strikes. The sequel to this serial was the 1950 Flying Disc Man from Mars, which used much of the footage from the original.

Plot
Astronomer Cyrus Layton is working late one night on his new airplane design in his observatory. He witnesses what he believes is a meteorite landing in the far distance. He contacts his niece Sheila and asks her to bring Craig Foster to the observatory to help analyze his discovery; he then sets out to search for the meteorite crater. Layton instead discovers a crashed spaceship; the ship’s pilot emerges and explains that he is from the planet Mars.

Mistakenly thinking the alien is friendly, Layton takes him back to the observatory. Once there the Martian, calling himself “The Purple Monster,” wishes to see Layton’s designs for the new airplane/spaceship. He proudly shows the alien his designs until the alien explains that he is now stealing them, to build a spaceship for himself to fly back to Mars, where a fleet of the ships will then be used invade the Earth. When Dr. Layton objects, the Martian murders him with a weapon that emits a “carbo-oxide” gas, which kills instantly. The alien then transforms into a ghost and takes over Dr Layton’s body. Doing so fools the astronomer’s niece Sheila and criminologist Craig Foster, both of whom work with Dr. Layton’s foundation, which is responsible for commissioning the spaceship project.

Inhabiting Dr. Layton allows the Martian to witness the unrelated theft of the plans by a gangster named Garrett. The Martian convinces Garrett and his gang to aid in the invasion plot. With the criminals’ help the alien begins building the spaceship. Eventually, however, the Martian’s efforts at pretending to be Dr Layton fall apart, and Foster and Sheila realize what is happening. A series of action scenes show the pair trying to figure out and stop whatever the alien is doing on Earth. Craig and Sheila constantly battle the Purple Monster’s henchmen, who use mind-control poisons, carjackings, and even a booby-trapped vacant lot to dispose of Craig and Sheila.

The closest the criminals come to succeeding is in Chapter 7 (“The Evil Eye”), when Sheila is lured into a trap at the gang’s hideout. Foster gets the information out of a captured gang member and speeds to the house to save Sheila, who has been tied up and gagged inside a room filled with explosives set to detonate after an electric eye is tripped.

At the end of Chapter 7, Foster steps into the electric eye, triggering the explosives and detonating the building. However, at the beginning of Chapter 8, Shelia manages to remove her gag and alert Foster about the eye, allowing him to jump over it. Once safely out of the building, Foster shoots a henchman, causing him to fall into the electric eye, triggering the bomb.

In the last chapter Craig and Sheila realize that the Purple Monster is using Professor Layton’s body; they devise a plan to uncover the truth. While Sheila gets the supposed Doctor Layton to come downtown to sign some papers needed for funding, Craig slips into Layton’s office and secretly installs a movie camera which will be remotely activated when the telephone is used. Foster then escapes and calls the office to advise him that he will be bringing reinforcements to search the observatory, which he has discovered is the Purple Monster’s hideout. Craig and Sheila arrive to find the observatory deserted. Sheila goes to the basement where she stumbles upon Purple Monster’s subterranean lair and is kidnapped. Foster goes to check on Sheila and finds the basement empty. He then discovers the secret lair where Sheila has been bound and gagged. The Purple Monster orders his henchmen to dispose of her and destroy the observatory once he escapes.

The story ends with Craig Foster using a part of the spaceship, a sonic pulse cannon used to shatter meteors. He destroys the alien spaceship with the Purple Monster inside as he attempts to fly back to Mars to lead an invasion fleet against Earth.

Cast
Dennis Moore as Craig Foster. The hero was known as Carry Foster even as far into production as the final shooting script. The name was pencilled out for filming.
Linda Stirling as Sheila Layton
Roy Barcroft as The Purple Monster. The villain was originally called “The Purple Shadow”, which was also pencilled out for filming.
James Craven as Dr Cyrus Layton
Bud Geary as Hodge Garrett, Henchman
Mary Moore as Marcia
John Davidson as Emperor of Mars
Joe Whitehead as Carl Stewart
Anthony Warde as Tony, the Organ Grinder