The plot is a simple one: Andy Dufresne (played by Tim Robbins) is serving a life sentence for a crime he did not commit. While in prison he befriends an old convict (Red, played by the incomparable Morgan Freeman), also serving life for murder, albeit one that he did commit.
But The Shawshank Redemption is much deeper than a mere prison film, or a buddy movie. It's a film about hope and redemption, hardly surprising given the film's title. But it's not just about the obvious, easy-to-spot redemption of Dufresne, who escapes from Shawshank Prison and takes down the corrupt prison warden on his way out. It's more so about the redemption of two souls--Dufresne and Red. This element is what makes Shawshank, in my opinion, a truly great film.
Dufresne may be innocent of the crime of which he is accused, but he is also a "cold fish," as his fellow inmates call him, an opaque, distant soul whose unfeeling demeanor casts him in a bad light with the judge and jury, and lands him in prison. At the movie's outset he remains distant, uncommunicative, and reclusive. He theorizes (probably correctly) that this flaw is what drove his wife into another man's arms, and the situation that resulted in her death: "My wife used to say I'm a hard man to know, a closed book. She complained about it all the time...I loved her, I just didn't know how to show it, that's all. I killed her Red--I didn't pull the trigger, but I drove her away. That's why she died, because of me, who I am."
Red on the other hand is guilty of committing a senseless crime as a youth. But while he's served his time and is a worthy candidate for parole, he's not truly ready for release because, until he meets and befriends Dufresne, he doesn't appreciate what life has to offer. His appeals before the parole board twice in the film are hollow and unconvincing, and result in rejection. He is, in short, without hope, an old, institutionalized man who feels his only purpose is to go on living on the inside, swapping contraband for cigarettes.
It's amazing to watch how both men bloom and grow in one another's company as the film progresses, even in the midst of the hell of Shawshank. The prison is depicted as a vision of the underworld in the opening sequence, with the new convicts ("fresh fish") stripped naked, deloused with burning powder, and marched into dark, isolated cells where other inmates bet on who will break first. Guards beat prisoners mercilessly, even to death at times. Presiding over this hell is warden Norton, who, much like Satan, thinks himself God, but is corrupt and evil at the core.
In the end, both men face a simple choice (as told beautifully by Dufresne in the film's iconic line): It comes down to a simple choice, Red: Get busy living, or get busy dying. Confronted with the choice of suicide at the end of a rope, or taking a terrible risk for the chance at salvation, Andy chooses the latter, and his escape from Shawshank is a thing of cinematic beauty. He crawls through 500 yards of foul-filled sewer pipe (an apt metaphor for life, perhaps) before he escapes, throwing his arms up to heaven in praise.