This was a post apocolyptic, alien, nuclear war sciFi type thing, late 60’s I’d guess. We are made aware that somehow most/nearly all of the world has been annihilated, whether by aliens or nuclear war I can’t recall. The world as whole is deadly radioactive and populated by radiation loving aliens. The film focuses on the small group of human survivors, living within some sort of microcosm (an island I believe) that was somehow spared and is protected from the radiation and aliens. They live in fear of rain, as they believe it will bring radiation and the aliens will follow. The radiation and aliens are somehow vanquished (I think maybe it rains and it actually kills the aliens), and the outro scene focuses  on the hopeful young attractive couple who are eager to do their part to repopulate the world. I think the final shot is them holding hands and the screen reading “the beginning”….