A man's vision for a utopian society is disillusioned when travelling forward into time reveals a dark and dangerous society.
The Time Machine (1960)
A scientist and tinkerer builds a time machine and uses it to explore the distant future where there are two races, a mild gentle race and a cannibalistic one living underground.
His machine is stolen by the underground race and he must risk being captured (and eaten) to return to his own time.