Ex-army major Paul Krenner, assisted by his girl friend, Laura Matson, engineers the prison break of safecracker Joey Faust. After Laura brings Joey to an isolated, old ranch house, Krenner explains to Joey that he wants him to steal nuclear fission materials from a government facility for an experiment he is conducting. Krenner threatens to kill Joey if he refuses to cooperate, then takes him to a laboratory within the house. There, nuclear scientist Dr. Peter Ulof demonstrates a ray machine that renders a guinea pig transparent then returns it, unharmed, to visibility. Krenner explains that he plans to use the ray to make Joey invisible to the facility guards. Later, Joey discovers that the lab contains a lead-lined safe in which the highly volatile fission materials are stored to protect them from exposure to the rays, which would cause a nuclear explosion. That night, Ulof, a European refugee, tells Joey that during World War II he was forced to perform experiments on hooded patients, one of whom he discovered was his wife. After the war, Ulof fled Europe with his baby daughter Maria and became involved with Krenner, who is coercing him to participate in the experiment by holding Maria hostage and threatening to kill her. When Joey tries to free Maria, he is stopped at gunpoint by Laura. Later, Joey suggests to Laura that she join him in a bank robbery, utilizing the ray’s transparency inducing abilities, but she declines. After Ulof succeeds in making Joey transparent, Joey demands $25,000 to open the government's nuclear vault. Krenner is forced to agree to his terms, and after bewildering the facility’s guards with his invisibility, Joey removes the nuclear material from the vault and delivers it to Krenner. Later, Ulof informs Krenner that the guinea pig has died as a result of overexposure to radiation, leading him to believe that Joey will also die. Nevertheless, Krenner orders Ulof to continue working on Joey and articulates his goal of rendering entire armies invisible. When Krenner has Laura drive the once more invisible Joey back to the facility, Joey tells her that he intends to rob a bank instead. Joey enters the vault area of a bank and takes a sack of cash but, before he can leave, his invisibility begins to wear off and parts of him suddenly become visible. Although Joey escapes, he has been recognized and, as they return to the house, they hear a news broadcast over the car radio reporting that the police are looking for him. While considering how best to take refuge, Joey suddenly becomes invisible again. He enters the house, where he finds Krenner preparing to flee. After freeing Maria, Joey locks Krenner in her room, then, becoming visible once more, begs Ulof to return him to normalcy. Ulof agrees on the condition that he and his daughter be allowed to leave immediately. Meanwhile, Julian, Krenner’s guard and henchman, has captured Laura and now prevents Joey, Ulof and Maria from leaving. Laura then informs Julian that Krenner had bought his loyalty by promising to help free Julian's son, who is imprisoned in Europe. When Laura tells Julian that in truth his son is dead, Julian surrenders his rifle. Leaving Laura in the house, Joey and the others head outside where Ulof reveals that Joey has radiation poisoning and will live for only another month or so. After Ulof persuades Joey to destroy Krenner and his insane plan, Joey reenters the house while the others drive off to summon the police, Krenner escapes from the locked room and kills Laura. Armed with Julian’s rifle, Joey chases Krenner into the lab, where Krenner breaks a bottle of acid on Joey’s arm and points the ray machine at him. As they struggle for possession of the nuclear material, the proximity of the material to the ray causes a massive explosion, killing them both. Later, as authorities deal with the fallout from the explosion, Ulof confides to a federal agent that the government’s intelligence agency has discussed with him the possibilities of making agents and armies invisible. Ulof, however, fears that the invention could be stolen and used against them and believes it would be better that the secret formula die with Krenner and the self-sacrificing Joey.