I wrote this back in 1999 and still mostly agree with the (brief) analysis and conclusions given at the end. But see a few comments appended after the review.
"Forbidden Planet", released in 1956, is a well-done science fiction movie which succeeds at entertaining us, piquing our scientific and technological curiosity, and in addressing fundamental philosophical questions. It stars Leslie Nielsen (in a serious role) as Commander Adams, the commander of a starship assigned to look for survivors of a earlier survey ship sent to the Altair star system; Walter Pidgeon as Dr. Morbius, a philologist originally on the survey ship; and Anne Francis as Altaira, daughter of Morbius. The movie also introduces Robbie the Robot.
Note: this review gives away all major plot details.
The story opens with Commander Adams' spaceship entering the Altair star system and approaching the fourth planet, Altair-4. The movie reveals its age when one of the crewmen, on seeing the planet from space, remarks, "The Lord sure makes some beautiful worlds." The spaceship makes contact with Dr. Morbius who warns them not to land, but they land anyway to carry out their orders. They are escorted to Dr. Morbius' house by Robbie the Robot, a polite and courteous creature who can synthesize solid lead and frilly women's clothing with equal ease. On meeting Morbius, the visitors from space learn that he and his daughter Altaira are the only survivors from the survey ship, the rest having been destroyed by some "mysterious planetary force".
The obvious question of how Dr. Morbius and his daughter escaped destruction is posed but not answered until the end of the movie. As Commander Adams and his officers continue to interact with Dr. Morbius, Morbius reveals the secret of Altair-4: the planet was populated long ago by a highly advanced species, the Krell, which left vast subterranean self-maintaining machines in place, yet of whom themselves no trace exists. The special effects here, though over 40 years old, still convey a sense of the vastness and grandeur of the creations of the vanished aliens. Simultaneously, the affections of Altaira become a prize sought by the woman-hungry officers of the starship. Although the sexual tension created by the men and innocent Altaira is naive by today's standards, it is nonetheless credible and essential to the plot.
As the crew of the starship settles in on Altair-4 to await further orders from Earth, violent acts of increasing ferocity are committed by some kind of monster against the equipment and then the men of the ship. When called to offer some explanation for the attacks, Morbius can offer none but only recount that the same "pattern [of destruction] is being woven" as before. We learn that Dr. Morbius' mind was artificially expanded by a Krell machine to which he had access.
All the pieces are put together as the ship's doctor gives himself the same artificial brain boost. The ship's doctor realizes that the Krell had succeeded in one vast, ultimate technological triumph, powered and implemented by the vast underground complex of machinery. They had developed the capability to materialize matter in any form at any place purely from their thoughts. The doctor grasps the frightening implication that this capability, being implemented by machines, could be for any purpose, whether good or evil. It now becomes clear that the Krell, having developed this capability, allowed their inner lusts and hatreds to destroy themselves.
But how could the same force of destruction be acting against the crew of the spaceship, since the Krell themselves have been dead for centuries? This question is answered as a by-product of Altaira's relationship with Commander Adams, which has turned into love. By allying herself with Commander Adams, she has implicitly distanced herself from her father. When she becomes a target for the destroying monster, the puzzle is complete: her father Dr. Morbius is, by the power of his artificially expanded mind powered by the alien machinery, sending out the monster to destroy anything and anyone which threatens his sovereignty and ego.
In the final climactic scene, Dr. Morbius realizes that the monster is in fact his "evil self" and utters, with horrible realization, that he has "no power to stop it". He stops it by confronting it and denying it, which is in fact a form of suicide. The story closes by the destruction of the entire planet, triggered by Morbius.
"Forbidden Planet" addresses, on the one hand, man's ability to create noble and powerful technological marvels and, on the other hand, the question of whether and how this ability to create fundamentally alters the nature of man. The movie answers the latter in the negative: even a mighty race such as the Krell, vastly beyond the development of humanity on Earth, could not overcome their own inner evil and, rather, used their superior talents and knowledge toward destruction and death. The point is amplified by noting that the inner evils within a person are largely invisible to the person; Morbius had no idea until the end that it was his lust for power and importance which led to the death of his fellow crewmen. Further, when one is made aware of one's true inner nature, one realizes the impossibility of controlling or taming the evil within; Morbius saw that the evil within him had a life of its own and was beyond changing, because it was a direct expression of his own heart! The only answer given by the movie is that of self-destruction, i.e., one can destroy the evil within only by destroying oneself altogether.
Apart from the redemptive work of Christ, the only way to destroy the evil within us is to destroy us, since the evil is who we at base are: "If you then, being evil, know how to give good gifts to your children, how much more will your Father who is in heaven give what is good to those who ask Him!" Matthew 7:11
The movie makes the point as well that technological progress, along with the education and knowledge that are presupposed by it, cannot alter the fundamental base reality of our hearts. This point is being appreciated by contemporary postmodern audiences, but comes across clearly in "Forbidden Planet" which is over 40 years old. That true redemption of the person can happen through Christ is the reality we must seek to communicate to our friends and neighbors.
I think the statement above 'the evil is who we at base are' is too simple. Jesus calls us evil in the Matthew 7 quote, yet elsewhere affirms that we continue to retain the image of God. Still, evil has permeated and affected our being and every faculty, as the doctrine of total depravity affirms.