While California University of the Sciences professor Douglas McCadden (Ben Murphy) explores the tomb of the ancient Egyptian king Tutankhamun, an earthquake causes a wall in the tomb to collapse, revealing a hidden chamber. Inside, McCadden finds a mummy in a sarcophagus. Unbeknownst to McCadden, the "mummy" is not the body of a dead Egyptian, but an extraterrestrial alien in suspended animation, being wrapped up and buried alive thousands of years before and covered with a dormant, green fungus.
The body is brought back to California and McCadden has it examined by Dr. Ken Melrose (Austin Stoker) and X-rayed by student Peter Sharpe (Kevin Brophy) before a big press conference about the discovery. While reviewing the X-rays, Sharpe notices there are five crystals around the "mummy's" head. Sharpe steals the crystals and makes new X-rays to cover up his theft. He sells four of the crystals to students who are unaware of their origin. The second set of X-rays overdose the body with radiation. This causes the fungus to re-activate and the alien to awaken from suspended animation.
At the press conference the next day, one of the students touches the fungus on the sarcophagus, which eats away one of his fingers. The sarcophagus is then opened in front of the press to reveal that the mummy is gone. Melrose and his colleague Dr. Hayworth (Antoinette Bower) attempt to identify the fungus and destroy it.
At first, everyone assumes that the mummy's disappearance is because of a fraternity prank. However, University President Wendell Rossmore (James Karen) wants to pin the "theft" on McCadden, so that he can give the Egyptian department's directorship to his flunkie, Bruce Serrano (Sam Chew Jr).
Meanwhile, the "mummy" tracks down the students who have the stolen crystals. The crystals are crucial components of an intergalactic transportation device that will allow the alien to return to its home planet. The alien violently reclaims its crystals, and, when he brutally attacks a female student, Lt. Plummer (Darwin Joston) is called in to investigate the crime. As more students turn up dead or injured, Plummer believes that he is on the trail of a serial killer.
While Plummer conducts his investigation, McCadden translates the hieroglyphic text from the sarcophagus. He hopes it will reveal the identity of the mummy. The text reveals that Tutankhamun found the alien in a coma-like state. Thinking that the unconscious alien was a god, Tutankhamun and his attendants touched it and were killed by its infectious fungus. The king and the alien were then buried together in the king's tomb. McCadden, having figured out that the "mummy" is an alien, makes the connection between the alien and the crystals. He then traces the stolen crystals back to Sharpe, who admits to the theft and gives McCadden the one crystal he kept for himself.
In the end McCadden, Rossmore, Serrano, two students, a security guard and the alien all end up in a boiler room where the alien has set up its transportation device. The alien activates the device by placing the last recovered crystal on it; his mummy wrappings disintegrate, revealing his true form. The security guard urged by Serrano shoots at the alien, but McCadden leaps in front of the alien to protect it. As McCadden lies injured the alien takes McCadden's hand, and the two disappear. A single crystal is left where the alien stood. Serrano grabs the crystal, and the fungus begins to destroy his hand.
Time Walker was re-released for home video under a new title (and new opening and closing credits) in Spring 1991. This is the version that aired on MST3K.
Shout! Factory released it on DVD bundled with several other New World films: Lady Frankenstein, The Velvet Vampire, and Grotesque on September 27, 2011.
TV Guide rated it 1/5 stars and called it a cliched mummy film notable only for the non-traditional monster, which moves quickly. Stuart Galbraith of DVD Talk rated it 3/5 stars and wrote, "Though mercilessly ridiculed on Mystery Science Theater 3000, Time Walker is a naively charming low-budget horror and sci-fi thriller, a real throwback to a more innocent time."