A bunch of guys land on Venus for exploration, with their awesome robot. Like most cheap scifi flicks, they are attacked by plant monsters and lizard monsters for about seventy minutes and then they escape. But before escaping they find proof that the Venusians are humanlike (or at least were at some time) when they find a plate with a creepy carving of a human face on it.
The problem with this movie is that it is so slow. When the astronauts get to Venus it is pretty awesome- the surface of the planet looks relatively like the real thing (rocky and volcanic) and the only thing that causes modern man's sense of disbelief to crumble is that, well, people are walking around on Venus in these flimsy space suits. One guy gets a tear in his suit after fighting with some lizard men but this just kind of leaves him winded. 400+ °C will tend to do that to you. Oh, also there are lizard men.
So after an awesome ride on their robot across a lake of lava our heroes go home, without even really trying to investigate the singing voice they hear that just might be a woman! Or a monster. We will never know thanks to these cowards.
The part where they find the disc or whatever with the image of a human looking face carved in it is actually done pretty well and is even a little chilling. Just be prepared to go through what seems like an hour of boring talk on the spaceship before anything cool happens.