A mysterious hypnotist reverts his beautiful assistant back into the form of a prehistoric sea monster that she was in a past life.
| Actor | Role |
|---|---|
 Chester MorrisDr. Carlo Lombardi | Dr. Carlo Lombardi |
 Marla EnglishAndrea Talbott / Elizabeth Wetherby | Andrea Talbott / Elizabeth Wetherby |
 Tom ConwayTimothy Chappel | Timothy Chappel |
 Cathy DownsDorothy Chappel | Dorothy Chappel |
 Lance FullerDr. Ted Erickson | Dr. Ted Erickson |
 Ron RandellPolice Lt. Ed James | Police Lt. Ed James |
 Frieda InescortMrs. Chappel | Mrs. Chappel |
 Frank JenksPlainclothes Sgt. with Lt James | Plainclothes Sgt. with Lt James |
 El BrendelOlaf, the butler | Olaf, the butler |
 Paul DubovJohnny, the barker | Johnny, the barker |
| WWilliam HudsonBob, the ex-fiancé | Bob, the ex-fiancé |
| FFlo BertMarta, the Maid | Marta, the Maid |
| JJeanne EvansMrs. Brown | Mrs. Brown |
 Kenneth MacDonaldPolice Doctor with Prof. Anderson | Police Doctor with Prof. Anderson |
 Jack MulhallLombardi's Lawyer | Lombardi's Lawyer |
 Edward EarleProfessor Anderson | Professor Anderson |
 Bess FlowersParty Guest | Party Guest |
 Paul BlaisdellThe She-Creature | The She-Creature |