Now that Earth has developed nuclear weapons it is established that it is only a matter of time until nuclear powered vehicles can hit the outer reaches of space and threaten universal peace.
As a result, Klaatu and his robot Gort have come to Earth to ensure that Earth remains a peaceful planet but, because of international cold war, no-one will listen. As a result, he decides to force Earth to pay attention for to ignore him is at its peril.
Sure, the effects aren't up to much by modern standards - Gort's impregnable suit is clearly rubberised for the actor's comfort as he ascends the ramp into the spacecraft and the disappearing effect is very basic. But, as mentioned, this was probably groundbreaking for the beginning of the 1950's.
The moral message of the film is much clearer. The shock of the US army surrounding the unmanned spacecraft is still a prevalent image today, while one can't help thinking that Klaatu's attempts to bring together all the nations of the world would still be in vein, even now. A reason behind this ill-feeling is accounted for as well; by introducing the child to show Klaatu around Washington, director Robert Wise proves that it is nurture rather than nature that causes us to lose our innocence during adolescence.
Finally, the iconic scene where the film picks its title from is perhaps even more worrying nowadays; our reliance on electricity is so heavy. After the end of the film I watched an advert being shown in the UK from Citröen which encourages us to switch to using electric cars. Sure, it's efficient, cheap and easy... but what if it was turned off?