CIA Agent Max Reed witnesses the murder of Dr. Tetsu Segawa, a researcher for the mysterious Chronos Corporation. Dr. Segawa had stolen an alien device known as “the Guyver” from Chronos, but he hid it among a pile of garbage by the Los Angeles River before his death. Lisker, leader of the thugs that murdered Dr. Segawa, returns the metal briefcase to Chronos' president Fulton Balcus, only to discover that it contains an old toaster. At a dojo, Reed notifies Dr. Segawa's daughter Mizki of the incident while her boyfriend Sean Barker struggles to pay attention in class. Sean follows Reed and Mizki to the crime scene; there, he stumbles upon the Guyver unit stored inside a lunch box and stuffs it in his backpack. On his way home, his scooter breaks down in the middle of a back alley before a gang corners him. While Sean is being attacked by the gang, the Guyver suddenly activates and fuses with him. Sean, in his newly armored form, dispatches the gang members, but is shocked by his physical appearance before the armor quickly disappears into two scars on the back of his neck. The next night, Sean goes to Mizki's apartment and discovers his sensei murdered and Mizki abducted by Lisker's thugs. With the help of Reed, Sean rescues Mizki before the trio are chased by Lisker's gang of Zoanoids. They are trapped in an abandoned warehouse, where Lisker's thugs hold Mizki captive and Sean once again transforms into the Guyver to battle them. Sean defeats the Zoanoids before squaring off against Lisker. During the fight, Sean executes a headbutt, which temporarily malfunctions the armor's Control Metal. He kills Lisker's girlfriend Weber, but mistakenly knocks Mizki unconscious before the Zoanoids gang up on him and Lisker rips the Control Metal off his forehead, disintegrating the armor and seemingly killing Sean.
Mizki wakes up at Chronos headquarters, where Balcus shows her a gallery of Zoanoids before questioning her on how Sean was able to activate the Guyver. Dr. East, the head of genetics research, discovers that the Control Metal is regenerating itself into a new Guyver unit. After seeing Reed being experimented on, Mizki assaults Balcus and takes the Control Metal, threatening to throw it into the disposal chamber. In the middle of the ruckus, the Control Metal is flung off her hand and accidentally swallowed by Dr. East before it bursts through the Zoanoid's body and once again becomes the Guyver who says he has been rejected by death. Sean and Mizki free Reed from the experimental chamber before Sean once again battles Lisker and kills him. Before the trio proceed to escape, Reed suddenly mutates into a Zoanoid and dies due to his system rejecting the new form. Balcus reveals his true form as the Zoalord and corners Sean, but the Guyver's defensive system activates the Mega Smasher cannons on his chest and obliterates Balcus and the laboratory. Sean deactivates the Guyver armor before he and Mizki leave Chronos headquarters as Reed's former partner Col. Castle and the Zoanoid thug Striker look on.Jack Armstrong as Sean Barker/The Guyver
Mark Hamill as Max Reed
Vivian Wu as Mizki Segawa
David Gale as Fulton Balcus
Michael Berryman as Lisker
Jimmie Walker as M.C. Striker
Peter Spellos as Ramsey
Spice Williams-Crosby as Weber
Willard E. Pugh as Col. Castle
Jeffrey Combs as Dr. East
David Wells as Dr. Gordon
Linnea Quigley as Scream Queen
Greg Paik as Dr. Tetsu Segawa
The American version of the film, distributed by New Line, cut several scenes to focus more on action than humor. Producer Yuzna expressed confusion at some of the choices.
Glenn Kenny of Entertainment Weekly said the film features “surprisingly convincing costumes and effects, inspired casting, and energetic direction, [but] what sinks it is its unfortunate adherence to the time-honored direct-to-video clichés: an unearned paycheck for a onetime A-picture star, and a tendency to fall back on lame humor whenever the going gets slow.”
David Johnson of DVD Verdict criticized the film's "ham-fisted over-acting", "ludicrous plot contrivances", and "nauseatingly hokey soundtrack." Johnson called the film "a big, dumb joke" and said: "Despite some good creature effects, the movie crashed and burned and crashed again, weighted down by preposterous acting [and] corny music."
Nathan Shumate of Cold Fusion Video Reviews criticized the film, in particular “the annoying demeanor and lack of personality” of lead actor Jack Armstrong, adding: "If there ever was a movie made for fan appreciation only, this is it, [...] but not everything can be blamed on audience unfamiliarity; there are plenty of elements in this movie that don’t work even by fanboy standards.”
The film generated enough interest for a sequel, Guyver: Dark Hero. Armstrong was replaced by David Hayter in the role of Sean. The film was more well-received critically than its predecessor. The original film poster is commonly used as an example of deceptive advertising; the shot is composited to imply Hamill is the Guyver, rather than having only a minor part.