As Bapak said the entire story is based on Aliens taking over Earth but they do this by taking over the bodies of the recently deceased as their first 8 plans to take over had failed - hence the Plan 9 in the title. It had great special affeccts including using hub caps as flying saucers and burning paper plates to give the impression of them burning.
Not only did an actor take over the Bela Lugosi role but whereas Lugosi was small and dark, the person who took over (who I believe was director Ed Wood's wife's chiropedist (?)) was considerably taller and fair. But for me the best performance comes from Tor Johnson playing the police chief. Johnson was a Swedish wrestler who could hardly talk, let alone speak English. You really have to see it - probably a couple of times to see all the really bad mistakes. If you do watch out for the wobbly gravestones, the spaceships and what one cop does with a loaded gun!!
Ed Wood is generally accepted as the worse director who ever made a film and was a infamous cross dresser. He even made a film called Glen or Glenda in 1953 based on this (the tag line was "He Loved Women So Much, He Dared To Dress Like One!") and there was a 1990's documentary called Look Back In Angora
There was a film made in 1994 based on his life called (amazingly) Ed Wood. It starred Johnny Depp in the title role and Martin Landau as Lugosi and also Bill Murray Sarah Jessica Parker and Patricia Arquette. It directed by Tim Burton.
It won several awarRAB including the best supporting actor Oscar and a BAFTA nomination for the same for Landau. The IMDB entry is
here.
You may have a problem getting hold of Plan 9 (it's also called Grave Robbers from Out Of Space) but I would throuoghly recommend the "Ed Wood" film.