Alien lands to help prepare arrival of more aliens, who intend to dissuade humanity from furthering its atomic aims. Of course, we all act like obnoxious idiots, but unfortunately we don't get vaporized by the Venusians.
Workmanlike British version of The Day the Earth Stood Still, with shades of Devil Girl From Mars (mysterious extraterrestial visitor in a film basically taking place in one room of an inn) thrown in. Not bad, but turgid and very stage-bound.
Some commenters have complained about the American cars in a British setting, but it seems the filmmakers intended the film to be taking place in no a sort of Everyman Land, with no specific country in mind.