A while back, I posted a guest column on Mightygodking.com about the thematic linkages between late-70s sci-fi movies The Omega Man, Logan's Run, and Soylent Green. Someone posted in the comments section that they were actually hoping to see "the contrived and hilarious explanation of how these three movies were actually a trilogy."
To be honest, I just thought that was too obvious to post at first. Clearly, the radical overpopulation of Soylent Green eventually turns humanity into the breeding ground for a new supervirus, a radical plague that wipes out most of the human race and turns many of the rest into mutant vampire-like creatures. Only Charlton Heston, who narrowly survived his injury, escapes the plague's deadly effects. (Clearly, the fact that he has a different name and profession speaks to a fascinating "film between the films" showing his failed revolution and subsequent adoption of a new identity.)
At the end of The Omega Man, of course, Heston sacrifices his life to find a cure for the younger generation. He succeeds only too well--without an older generation to provide stability and tradition, the young people decide that there's no need for a post-thirty humanity, and they turn to scavenging pre-plague technology to create a futuristic Utopia for themselves. Which works fine, unless you decide to run.
And of course, somewhere above all the chaos, Bruce Dern silently watches and guards Earth's precious natural resources...