If you know nothing else about the movie 'Soylent Green,' we're guessing you know the twist ending. And if you don't, today is your lucky day, because we're going to spoil it for you, right here, right now.

Soylent Green is people.

There. Happy? I am -- because now, with that out of the way, we can talk about the rest of the movie. And it's a movie well worth talking about.

Released in 1973 and based on the Harry Harrison science fiction novel 'Make Room! Make Room!' 'Soylent Green' is set in the far-off future of 2022, when the world's population has gotten way, way out of hand. (The city of New York, for example, where the movie takes place, is home to 40,000,000 people.) That means everything is at a premium -- livable space, food, clean air, fresh water, you name it.

As the movie begins, police detective Frank Thorn (Charlton Heston, sealing his reputation as the go-to guy for dystopian futures) is investigating the death of a rich lawyer and stumbles onto a secret conspiracy that reveals just how screwed up the world really is. Now, if you know the twist ending (and thanks to that second paragraph, you do) the conspiracy isn't hard to figure out. But, as I said, that doesn't really matter. The mystery angle of 'Soylent Green' isn't why you watch. The reason to see this movie is because it creates one of the most convincingly bleak worlds in cinema history.

How bleak? How's this: Even though he's a cop investigating a murder, Thorn sees nothing wrong with grabbing a pillowcase and stealing everything he can from the dead guy's apartment, including a can of strawberry jam worth $150. He takes it back to share with his friend, Sol Roth (Edward G. Robinson), who's old enough to remember the good old days, when smog didn't choke the air and it wasn't always summer.

Thorn also meets Shirl (Leigh Taylor-Young), who was a sort of upscale sex slave to the dead man. Known as "furniture," she stayed with the apartment, hoping the next owner would want to keep her around. Desperate and degrading? Sure, but it beats living on the mean, crowded streets of New York.

Heston's very good in 'Soylent Green,' barreling through the movie with the crude energy of a guy who never knew a better world and doesn't appreciate how awful the one he lives in is. But Robinson is the heart and soul of the film, playing a smart man crushed to see everything worthwhile die off. It's he who discovers the secret of Soylent Green, and it's a lot worse than it just being made of people. (After all, there are billions of extra people around to make those bland looking green chips.)

Roth learns that though the corporations claim Soylent Green is made from plankton, it can't be. There is no more plankton. The seas are dead. Learning this is too much for him to bear, and he visits a local suicide parlor, where friendly attendants help him die, surrounded by beautiful music and images of a nature-filled world that no longer exists.

Thorn arrives at the parlor, but it's too late, and he watches as his friend dies, weeping uncontrollably. It's hard to say why he's crying -- yes, his dearest friend decided life wasn't worth living, but that's not all of it. You also see that Thorn is both touched and devastated by the scenes of nature he never experienced. But that's not all of it, either.

Truth is, this was Edward G. Robinson's last film, after decades in Hollywood and more than 100 movies to his credit. No one knew Robinson was dying of cancer during filming (except for Robinson), but clearly he wasn't well, and Heston had been a friend since they co-starred in 'The Ten Commandments' 17 years earlier. All those elements, both fictional and factual, make it a powerful, heartbreaking scene -- all the more so because it's in the middle of such a strange, bleak movie.

That's because, when you get right down to it, it's not the twist ending or the mystery plot or the special effects that make this movie memorable. At it's heart, what really matters in 'Soylent Green' is ... people.