We open in the woods after dark during an intense storm and see the old Willows property out on Lake Marsh, roll credits. Wandering around out in this foul weather are Mac (Bud Osborne) and Jake (John Warren). A bolt of lightning strikes a tree bough and it falls off. They decide to make for the Willows place as a refuge from the storm. They think it is deserted. They knock on the door when they see signs it may not be vacant. Dr. Eric Vornoff (Bela Lugosi) answers the door and refuses them entry. When it appears that Mac and Jake may use their guns, Lobo (Tor Johnson) appears and the two think Tor is the "Monster of Lake Marsh" and quickly depart. Vornoff returns to his lab via the entrance behind the fireplace. He slips into his lab coat and proceeds with an electrical experiment. He peers into a window and a large octopus appears. Mac and Jake have made it to the lake shore. Mac falls into the lake and is attacked by the octopus. Jakes returns to help his friend. He tries to pull him out, but failing that, he uses his rifle to put a couple of bullets into the thing. Lobo appears and grabs Jake. He takes him back to the house and prepares him for one of Vornoff's experiments. When Jake wakes he's in the lab, strapped to a gurney with some electrical device on his head. Lobo moves some equipment over him. Vornoff explains to Jake that Lobo is mute. He can hear his pleas for help, but he cannot speak. Vornoff fires up his equipment while Jake struggles. Vornoff tells him, "you will be soon as big as a giant...with the strength of twenty men...or like all the others, dead!" Jake screams and dies in an instant. Vornoff examines him, but only confirms his worst fear."Monster Strikes Again!" screams the Daily Chronicle headline, and the Daily Globe shouts "Monster Takes Two!" At the police station, officer Kelton (Paul Marco) is questioning a drunk (Ben Frommer). The drunk is being very uncooperative. Two police offers drag him downstairs for booking. During the confusion, the newsboy (William Benedict) arrives and tries to go into the Captain's office with his newpapers, but is stopped by Kelton. Kelton starts to read one of the papers, but hears Captain Robbins entering his office from the hallway. Captain Robbins (Harvey B. Dunn) is sitting at his desk petting a bird. He tells Kelton to send Lt. Craig in to see him. Robbins peruses the newspaper headlines. While waiting for Craig to arrive he gives his bird a drink of water. Lt. Dick Craig (Tony McCoy) arrives, sits down and they discuss the Lake Marsh disappearances, twelve in the last three months. They examine the recovered remains of Jake and Mac, a gun and coat. Janet Lawton (Loretta King) bursts into Robbins' office with Kelton making a valiant effort to deep her out, but failing. We learn that Janet and Dick are engaged. They discuss her newspaper story. When she gets no new information, she tells them she intends to drive out to Lake Marsh and do a little of her own investigating. She drives back to the newspaper office.Janet makes her way to the file room and talks to Tilly (Ann Wilner) about the old Willows place out on Lake Marsh. She finds what she is looking for in the files. On her way out she bumps into Margie (Dolores Fuller) who warns her that the boss is looking for her. The police called the papers owner and told them they want a stop to the monster stories because of public panic. Janet gets back in her car and heads for home.Captain Robbins is meeting with Professor Strowski (George Becwar). Lt. Craig enters Robbins' office and is introduced to the professor. Strowski offers his assistance in the investigation. After he departs, Robbins warns Craig that Strowski should be watched. Janet departs for Lake Marsh as the weather starts to change. In a rainstorm near the Willows place, she blows a tire and crashes her car. Dazed, she exits her car and notices a snake in a nearby tree and faints. Lobo arrives at the scene of the accident, and picks her up and takes her back to the house. Janet is asleep on a couch and wakes to the sight of Vornoff. Vornoff hypnotizes her into a dream state. Martin (Don Nagel) and Lt. Craig are driving in their police car and stop. They decide to take the road to the Willows house. Strowski was supposed to meet Craig at 10:00 a.m., but failed to show for the appointment. They drive a short distance and stop when Lt. Craig spots Janet's car, but no trace of her. They head back to a coffee shop to see if she stopped there. Strowski arrives on the same road just after Craig and Martin leave. He starts walking through the woods. Martin and Craig arrive at the Café and call police headquarters. Craig is told to find Strowski. Janet is not at home or at work, and no one has seen her.Back at the Willows place, Vornoff and Lobo are coming into the lab with Janet's breakfast. When Janet sees Lobo she recoils in fear. Vornoff tells her, "don't be afraid of Lobo, he's as gentle as a kitten." Lobo is dismissed, but lingers to stare at Janet. When he fails to comply, Vornoff whips him. Vornoff returns to talk to Janet. He introduces himself, and tells her she had an automobile accident. She concludes she is at the old Willows place based on her research. Strowski finds the house and approaches while Vornoff hypnotizes Janet. He then directs Lobo "to take the girl to my quarters." Strowski enters the house quietly, looks around then enters the living room. He places his briefcase on the mantle and is greeted by Vornoff. Strowski is there to convince Vornoff to return to their country to continue his research. Vornoff, having been exiled twenty years earlier, is in no mood to return or to share his research--the creation of a master race. When Vornoff informs Strowski he has no intention of returning, Strowski pulls a gun and tells him that their government ordered his return. Lobo easily disarms Strowski and takes him down to the lab and throws him to the octopus.Martin and Craig, driving along the road to the Willows place, discover the abandoned rental car Strowski was driving. Martin and Craig split up, Craig walking to the Willows place. He takes the same path Strowski took earlier and falls into some quicksand. He shoots at some alligators and manages to escape the quicksand.Captain Robbins goes to the newspaper office to talk to Tillie. She was the last person known to have spoken to Janet. She informs him that Janet came in around 2:00 p.m. and left about 3:30 p.m. She was researching the file on the sale of the old Willows place. Robbins calls police headquarters and asks for all information on Dr. Eric Vornoff.At the lab, Vornoff summons Janet, now dressed in a wedding gown (!?) He directs her over to the examining table. Outside Lt. Craig enters the house, looks around, then enters the living room and finds Strowskis hat. Vornoff commands Lobo to strap her to the table. When he hesitates, Vornoff whips him. Back in the living room, Craig discovers Strowski's glasses and the file Strowski was carrying on Vornoff. When he places the folder down on the fireplace mantle, it activates the doorway to the lab. He pulls his gun, and makes his way to the lab. Vornoff orders Lobo to get the equipment ready. Lobo has developed a strong attraction to Janet and does not want her harmed. Vornoff continues preparation to turn Janet into the "Bride of the Atom". Lt. Craig enters the lab and orders Vornoff to release her. Lobo quietly enters behind Craig and knocks him out.Robbins, Kelton and two police officers meet Martin on the road to the Willows place. They walk to the house. Vornoff has Craig chained to the lab wall and tells him Janet is the subject of his research. Vornoff switches on the equipment, and Lobo realizing Janet will be Vornoff's next victim, turns on his master and attacks. Vornoff shoots him with Craig's service revolver, but the bullets have little effect. Vornoff is knocked to the floor. Lobo releases Janet from the table. As Janet is trying to release Craig's shackles, Lobo carries Vornoff to the table and straps him down. Lobo fires up the equipment and Craig, now loose, tries to intervene. But the stronger Lobo easily knocks him out. Lobo returns to the equipment and Vornoff ends up the subject of his own experiment--successful this time.Robbins and Martin enter the house, look around, enter the living room, and find the same clues as Craig. Vornoff, now an atomic superman, attacks Lobo who falls against some electrical equipment and is electrocuted. It starts a fire. Vornoff now mutated, grabs Janet and leaves the lab. Craig awakens and follows the pair out of the burning building. Robbins and Martin see the smoke, and also escape the building. Kelton sees Vornoff some distance away carrying Janet, with Lt. Craig in hot pursuit. The house is now fully engulfed in flames. Vornoff puts Janet down to make his escape easier. The police fire at him, but the bullets have no effect. Lt. Craig rolls a boulder onto him, which knocks Vornoff into the lake where he is attacked by his own atomic mutation, the octopus. Craig returns to Janet, who is ok and still wearing her wedding dress. Vornoff and his octopus are struck by a bolt of lightning which results in an atomic explosion. We close with Captain Robbins, joining Janet and Lt. Craig. Robbins opines, "he tampered in God's domain."
Output: Question: What mutated creature does Mac confront after falling in the lake?, Answer: An octopus.