Plan 9 From Outer Space (1959)

Members of the Los Angeles Police Department, led by Inspector Clay, investigate what look to be the brutal murders of two gravediggers at the cemetery where they worked. These deaths coincide with sightings of flying saucers not only over Los Angeles but over other areas of the United States. One who saw them was Jeff Trent, a commercial pilot, when he was in flight, he sworn to secrecy about what he saw by the United States Army. What Trent and the public at large are unaware of is that the army not only knows first hand about these latest sightings but have had tried to shoot down flying saucers in the past. The alien beings, who have been trying to make contact to dissuade the government from developing what would eventually be a doomsday device which would destroy the universe, are now resorting to what they call Plan 9: the revival of human corpses which in turn are to kill humankind before they destroy the universe, hence the focus on the cemetery. People like Trent may be the ones who will have to save both humankind and the universe from what is happening between the alien beings and the government authorities.