Silent Running (1972)

Freeman Lowell (Bruce Dern) is friend to bunnies and fauna. As one of four crewmen aboard a space freighter he waits for the day when his precious cargo of plant and animal life can be returned to a dying Earth.
A world without trees and flowers is a genuinely horrifying thought, so I'm with Bruce. However, he's a sensitive type whose reactions give rise to the question: did he take his obsession much too far?
It's a sci-fi by Douglas Trumbull that overcomes a limited budget by having a strong script and a fine actor able to bring out the nuances within it.
The faithful drones that aid the humans are adorable metal friends and will stay in your thoughts long after the credits have finished rolling.

3½ house rules out of 5