Spoiler Warning: if you have never seen this movie and you are unaware of the surprise ending, I recommend that you watch the movie first before reading this. There is nothing worse than watching this movie while being aware of the surprise ending.
As I alluded in the warning above, I already knew the secret of Soylent Green ("It's people!") because of a famous Saturday Night Live skit by Phil Hartman about its fictional sequel _Soylent Green II_ I saw a few years ago ("Soylent Green is still made out of people! They didn't change the recipe like they said they were going to! It's still people!!"
http://snltranscripts.jt.org/92/92osoylent.phtml).
The movie was much better watching it for the second time because I made my wife watch it and she was totally unaware of this movie and it's surprise ending. My wife is very cynical about "surprise" endings because she can usually figure them out 15 minutes into the movie. She didn't figure out the secret until she saw the Soylent Green factory and the way it processes dead people.
The premise of the movie is that the world is overpopulated and there are very few natural resources to sustain it. There is ecnomic collapse and food is very scarce. The film opens with a musical and visual montage showing the world in the 1800s where people sustained themselves with traditional farming and eventually more images of modernity creeping in, with automobiles, skyscrapers and then the music turns ominous showing riots, pollution and decay.
Charlton Heston plays a detective who is investigating the murder of the President of Soylent Corporation. He has a two-year backlog of unsolved murder cases and this is one murder he absolutely feels the urgent need to solve ("You know, there are 20 million guys out of work in Manhattan alone just waiting for my job."). He has an assistant who is very old and remembers how things once were ("People were always rotten. But the world 'was' beautiful.").
I think that the world described in this movie is very possible in the near future: severe competition for the few remaining jobs left, running water is a luxury, food riots and massive corruption. Even the main hero is not above this, Heston is found stealing all of the necessities of life wherever he goes and takes bribes. Whether or not the government will process dead people to mitigate the food shortages is beside the point, a world with resource shortages will be a very unpleasant place to live in.
My favorite scene in this movie is when Heston and his assistant enjoy a meal consisting of beef stew and some fruits and vegetables. This is a typical meal that most people today take for granted, yet the two characters make it seem that they are enjoying a fine meal in a fine restaurant. The movie is worth watching just for this scene alone.
Around the time I bought this movie on DVD, I also got Zardoz (1974) starring Sean Connery and Rollerball (1975) starring James Caan. I really liked Zardoz--it was the best of this "trilogy" of the future apocalypse. But the most realistic of the three has to be Soylent Green.