American International Pictures’ The Ghost in the Invisible Bikini (released 6 April 1966). The Beach Party Gang in a haunted house: the final (?) beach party movie, starring Pajama Party’s Tommy Kirk and Ski Party’s Deborah Walley.
“Hiram Stokely is visited in his coffin by Cecily, the ghost of his sweetheart of 30 years ago. She tells him that if he performs a good deed within 24 hours he can gain admittance into Heaven as a member in good standing and also become young again. The good deed is to prevent his scheming attorney, Reginald Ripper, from swindling Hiram’s rightful heir, young Chuck Phillips, out of his inheritance. As Chuck, his fiancée, Lili Morton, and his spinster aunt Myrtle Forbush prepare for the reading of the will, Myrtle’s swinging nephew Bobby arrives at the Stokely mansion to spend the weekend. Also on hand are Eric Von Zipper and his Rat Pack. Amidst all the mayhem, the villainous Ripper finds it difficult to carry out his plan for doing away with Chuck. When Ripper makes a last attempt at murder in the mansion’s basement chamber of horrors, Hiram and Cecily foil the scheme, and the inheritance rightfully goes to Chuck. Having done his good deed, Hiram is given his reward, but it is more than he had bargained for; as Cecily leads him toward Heaven, he becomes 3 years old again.” Turner Classic Movies.