Forbidden Planet is a 1956 science fiction film directed by Fred M. Wilcox, with a screenplay by Cyril Hume. It starred Leslie Nielsen, Walter Pidgeon, and Anne Francis. The characters and its setting were inspired by those in William Shakespeare's The Tempest and its plot has many similarities.
Forbidden Planet features special effects for which A. Arnold Gillespie, Irving G. Ries, and Wesley C. Miller were nominated for an Academy Award. It was the only major award nomination the film received. Forbidden Planet features the groundbreaking use of an all-electronic music musical score. It also featured "Robby the Robot", one of the first movie robots that was more than just a "tin can" and had a personality.
Early in the 23rd century, the United Planets Cruiser C-57D has been sent to the planet Altair IV, 16 light-years from the Earth. Its mission is to discover the fate of an expedition sent 20 years earlier to establish a colony there. The cruiser is contacted by Dr. Edward Morbius, who radios the crew and warns them to stay away. However, the starship's captain, Commander John J. Adams, decides to land on Altair IV.
The ship is met by Robby the Robot, who takes Adams, Lieutenant Jerry Farman, and Lieutenant "Doc" Ostrow to meet Dr. Morbius. Morbius explains that an unknown phenomenon killed nearly all of the other members of his expedition and destroyed their starship, the Bellerophon. Only Morbius, his wife (who died of natural causes), and his daughter Altaira, now 19 years old, survived. Morbius fears the crew of the C-57D will also meet the same fate. Altaira cannot recall any man but her father and barely remembers her mother. She is interested in learning about human relationships.
Morbius explains he has been studying the "Krell", the natives of Altair IV who, despite being far more advanced than humanity, all died mysteriously during a single night 200,000 years before — just as they had achieved their greatest triumph. Morbius shows Cdr. Adams and his party a device he calls a "plastic educator". Morbius explains that the captain of the Bellerophon had tried it and had been killed instantly.
Morbius used it but barely survived. However, he doubled his intellectual abilities as a result. He says this enabled him to build Robby and the other technological marvels in his house. Morbius takes them on a tour of a vast cube-shaped underground Krell installation, 20 miles [30 km] in each direction and powered by 9,200 thermonuclear reactors. This complex had been operating and maintaining itself ever since the extinction of the Krell. When asked about its purpose, Morbius says he knows only that it can supply a practically limitless supply of power.
One night, a valuable piece of equipment in Cdr. Adams's starship is damaged, though the sentries who had been posted spotted no intruders. In response, Adams commands that a defensive force-field fence be set up around his starship. However, this defense proves to be useless when whatever caused the damage returns, passes unseen, and unharmed through the fence to kill Chief Engineer Quinn. Dr. Ostrow is confused after examining the footprints that it left behind, saying that the creature appears to violate all known evolutionary laws.
The intruder returns again on the next night, and it is discovered to be invisible. Its appearance is revealed only in outline by the beams of the force field and the crewmen's weapons. Several men are killed by the monster, including Lt. Farman. Simulaneously in a Krell laboratory, Dr. Morbius is awakened from a pitched nightmare by a scream from Altaira. At that instant, the creature vanishes.
Later, while Cdr. Adams confronts Morbius at the house, Lt. Ostrow sneaks away to use the plastic educator, but he is mortally injured. Just before he dies Ostrow tells Adams that the underground installation was built to materialize any object that the Krell could imagine. However, the Krell had forgotten one vital thing: "Monsters from the id!". Morbius objects, pointing out that there are no Krell left. Adams replies that Morbius's mind — expanded by the plastic educator and thus able to interact with the gigantic Krell device — had created subconsciously the monster that had killed the rest of his expedition 20 years earlier—after they had voted to return to the Earth. Morbius scoffs at Adams's theory.
When Altaira declares her love for Cdr. Adams in defiance of her father's wishes, the alien monster of the mind comes after them. Dr. Morbius commands Robby to kill it, but the robot freezes, unable to harm a human being. Robby recognizes the monster as an extension of Morbius, and his only way to destroy it would be to kill Morbius; instead, the clash of orders burns out the robot's circuits. The creature breaks into the house and then melts its way through the nearly-indestructible door of the Krell laboratory where Adams, Altaira, and Morbius have taken refuge.
Morbius finally accepts the truth that the creature is an extension of his own mind, and he tries to renounce it. When Morbius is mortally injured trying to intervene, the creature disappears permanently. While Morbius lies dying, he directs Cdr. Adams to press a lever that sets the Krell complex to self-destruct. Adams, Altaira, Robby, and the rest of the starship's crew take off for outer space. From there, they witness the destruction of the entire planet of Altair IV from a safe distance away. Before the film ends, Adams says to Altaira, "We are, after all, not God."