We open with the credits amid an underwater panorama of sea life, this is followed by a pounding surf. We see Dr. Carlo Lombardi (Chester Morris) on the beach. He encounters a barking dog (King) and through his hypnotic power repels it.
Dorothy Chappel (Cathy Downs) arm in arm with her new boyfriend, Dr. Ted Erickson (Lance Fuller), who is a professor of Psychic Research, step outside on the terrace from a cocktail party at her parent's beach house. They go for a walk along the beach. We see Dorothy's parents, Timothy Chappel (Tom Conway) and his wife (Frieda Inescourt) out taking some air. Mrs. Chappel is concerned because Dr. Lombardi predicts something terrible will happen on the beach that very evening. Dorothy and Ted walk along the beach talking, as Lombardi enters a beach shack and discovers it is a shambles. He also discovers a dead couple. He seems neither surprised nor concerned. He discovers what appears to be seaweed on the floor. The dog approaches Dorothy and Ted and becons they follow him to the beach shack. They see Lombardi departing the house. Ted enters and sees the bodies, exits, and tells Dorothy to go home and call the police. He will remain there.
The police arrive. Lt. Ed James (Ron Randell) and his Sergeant (Frank Jenks) examine the crime scene. They also notice the seaweed. The sergeant thinks it is Lombardi's creature, or so he tells James that is his wife's conclusion.
Johnny (Paul Dubov) the carnie and general busybody runs a game of chance on a rundown pier arcade. We learn that he knew Andrea before Lombardi. He and Lombardi trade barbs. Lombardi goes to a small auditorium and we meet Andrea Talbott (Marla English). She is in a trance and Lombardi kisses her to wake her. He is in love with her, but she does not reciprocate. She hates and resents the power he has over her, and she tells him as much. There's a knock at the door. Lt. James and Ted arrive and confront Lombardi. Ted makes a positive identification of Lombardi as the person exiting the beach shack, the site of the recent Jefferson double murder. Lt. James questions Lombardi, then takes him in for further questioning.
Olaf (Ed Brendel) is serving Mr. Chappel coffee with his breakfast out on the terrace. Ted arrives, dressed for tennis. They discuss the Jefferson murders and a possible business venture based on same. They conclude Lombardi is somehow involved, but Chappel intends to use Lombardi as a moneymaking vehicle. Ted is very reluctant to get involved for both professional and ethical reasons.
Chappel goes down to the arcade to discuss business with Lombardi, but Lombardi anticipated his visit. They agree to a 50/50 arrangement, each for his own reasons. Lombardi returns to Andrea, and through her summons the She-Creature (Paul Blaisdell) and directs it to kill Johnny. It breaks down Johnny's door while he is relaxing on his cot, and kills him. The next morning, Lt. James returns to see Lombardi and arrests him for Johnny's murder, but Lombardi's lawyer (Jack Mulhall) springs him shortly after his arrest. The beaches are closed.
That evening a party at the Chappel house is well attended. Mrs. Chappel is excited that Lombardi will be entertaining. Olaf and his wife, Marta (Flo Bert) are the serving staff for the evening. They are employed by the Chappels as much for comic relief as their domestic skills. A very drunk Bob (William Hudson), former fiancée of Dorothy, tries to rekindle their former relationship, but she gives him the brush off. Andrea and Lombardi arrive, and all eyes are on Andrea. Dorothy reacts with a tinge of jealousy. Lt. James arrives almost immediately thereafter. He asks Chappel for permission to remain and watch the performance. Everyone is instructed to be seated. Lombardi proceeds with his hypnosis of Andrea. She tries to resist his commands, but eventually complies. Lombardi challenges Dr. Ted Erickson to debunk his work publicly during the demonstration. Ted watches Lombardi regress Andreas back to a past life. She details the life of one Elizabeth Anne Weatherby from the year 1618. She provides very accurate details of her life and times. Elizabeth is the spirit that Lombardi uses to summon the She-Creature. The spirit of Elizabeth is asked to open a window and close some drapes, which happen on cue. The crowd murmurs astonishment. King appears and growls at Lombardi, and he commands the spirit of Elizabeth to remove the dog, which it does. Lombardi predicts the creature is coming to the assembled guests and they depart the Chappel house. Mrs. Chappel faints. Lombardi grabs his hat and coat and departs with Ted close behind. The She-Creature appears and is about to attack Ted, but Andrea wakes and screams and the creature walks back to the ocean.
The next morning Dorothy and Ted talk of the evening's events. Lombardi was asked to spend the night as a guest of the Chappels. He joins them on the terrace. Mr. Chappel congratulates Lombardi on his performance. Lombardi informs Chappel he will be his house guest for a while. Lombardi meets with Andrea and informs her Ted will try to hypnotize her later at an arranged experiment and she should resist.
Ted and Lt. James meet in Ted's lab. He agrees to let Lt. James sit in on the meeting he will be having later with some colleagues to test Andrea. Andrea and Lombardi arrive, along with Ted's professional colleagues (Kenneth MacDonald) and Professor Anderson (Edward Earle). Lt. James is recording the session. As proof of his powers, Lombardi summons Elizabeth to remove Prof. Anderson's glasses, and they come off his face. Lombardi departs with Andrea.
The story is published and a million books are sold. Chapel tells Lombardi he is now rich and should find other lodgings, but again Lombardi refuses to leave. A tour of the country with Andrea follows to huge crowds.
The She-Creature kills a couple in their car as Lombardi and Lt. James look on. James is convinced Lombardi did it, despite being Lombardi's alibi. During the tour, Andrea and Ted have fallen in love. Lombardi is furious at this new development and tells her he will arrange Ted's death. Dorothy tries to reconnect with Ted, but he makes it clear to her he's now in love with Andrea.
Ted and Andrea walk along the beach with Lombardi spying on them. Lombardi directs King to attack Ted, but Andrea intercedes and calls the dog off. They head back to the Chappels house for the evening show. Lombardi tries to hypnotize Andrea but she resists. She eventually succumbs and falls into a trance, but is still uncooperative. Lombardi scares the crowd into leaving. He returns to Andrea to summon the She-Creature. Lt. James is on the beach and sees the creature materialize and shoots it, but it is impervious to bullets. It attacks him. Ted sees this and helps him to his feet and James tells Ted he now believes in Lombardi's power to summon the creature. The She-Creature, now set loose, attacks Chappel in his study. It then heads for the living room and Ted. But Andrea has now developed a little power herself, and directs the creature to Lombardi. After attacking Lombardi, she directs the creature back to the sea, where the police shoot at it. Lombardi, badly beaten, lives long enough to release Andrea from his control then collapses. Andrea, now herself, and Ted walk out on the terrace. We close with a shot of waves on the beach.