On their way to the cinema, Private Steve Rogers and his young friend Bucky Barnes witness an armed robbery in progress. After changing into their costumes in an alleyway, the pair emerge as Captain America and Bucky. Bucky gets knocked unconscious by a blackjack during the ensuing fight and so, once he has mopped up the crooks, Steve Rogers takes his injured friend to the nearest hospital in a taxi. The nearest hospital turns out to be a private establishment owned by Dr. Grimm, a bald, heavy-built man in a white smock. Grimm says that Bucky needs to stay at the hospital for several days while he recovers.
That evening, Bucky shares his concerns about the creepy hospital with his nurse, Miss Ray. She explains that she has only been working there for a few days since the last nurse left suddenly without giving notice. Miss Ray does not like the hospital, either. At night she hears strange noises.
During the night, Bucky is woken by loud cries. Convinced he can hear Miss Ray screaming, Bucky opens the door to his room and encounters a monstrous hump-backed man crouching in the corridor. Dr. Grimm arrives with a leash. ‘Don’t be afraid,’ the Doctor tells Bucky. ‘Igan is harmless when I am holding his leash.’ When Bucky says that he heard a woman screaming, Dr. Grimm insists it must have been an auditory hallucination brought on by the young man’s nerves. The Doctor warns Bucky not to leave his room at night as he allows Igan to roam around a little then.
The following morning, Bucky’s breakfast is brought by Dr. Grimm’s hideously ugly servant Lomm instead of Miss Ray. When Bucky asks after the nurse, Lomm says he knows nothing but perhaps she ran away during the night. In any case, the Doctor has already advertised for a new nurse.
By now convinced that something sinister is going on at the hospital, Bucky writes to Steve, saying he thinks Captain America should investigate the place. Dr. Grimm catches Bucky in the act of writing the letter and wraps his powerful right arm around the boy’s face. Nonetheless, the Doctor decides to mail Bucky’s letter so that Captain America can be lured into a trap.
After receiving Bucky’s letter, Steve Rogers steals away from Fort Lehigh, the army camp where he is currently stationed, wearing his Captain America uniform. Arriving at the hospital, Captain America wrestles with the monstrous Igan. After being punched, Igan retreats in order to pull a hidden lever in the wall. A steel door closes and the airtight room begins to fill with water. Captain America escapes through a trapdoor in the ceiling only to meet with Dr. Grimm. Holding the hero at gunpoint, the Doctor explains his grim purpose. He has created a powerful monster called Gorro which requires much energy to survive and so the Doctor draws energy from human captives. Bucky is currently being prepared as the next victim of this process. When Captain America lunges forward in anger, Grimm presses a concealed button and a glasso-metal cage descends, trapping the hero.
The next morning, Steve Rogers’ friend, the beautiful Betty Ross, applies at the hospital for the available position as nurse. After checking Betty is in good health, Dr. Grimm awards her the post.
Subsequently, Dr. Grimm marches Captain America at gunpoint to a laboratory where Bucky is strapped to a bed, surrounded by sinister-looking medical apparatus. Betty is also there with her hands tied behind her back by a rope that is secured to the ceiling. Enraged, Captain America whirls and punches Dr. Grimm before the fiend can react. The masked hero then frees Bucky but not before Grimm’s servant Lomm has managed to open the cage containing Gorro. A loud roar echoes from the far end of the room as the monster emerges. Captain America leaps over Gorro and knocks out Lomm. But then Gorro seizes the sentinel of liberty with huge beast-like fingers. Having freed Betty, Bucky shoots Gorro repeatedly with Dr. Grimm’s fallen automatic. Dr. Grimm meanwhile has pursued Betty onto a tower balcony and prepares to throw her to her death. After Captain America pulls Betty from the Doctor’s clutches, Grimm and the hero grapple briefly until the maniac plunges over the balcony to his death. Captain America explains to Betty that Dr. Grimm had aimed to develop Gorro to a point where he would become almost human. Aided by Gorro’s strength, Grimm planned a career of crime.
On returning to camp, Steve thanks the sentry, Pete, for covering for him. But he is mistaken. The ‘sentry’ is not Pete. It is Sergeant Duffy, who punishes Steve and Bucky with a month’s duty in the kitchen for leaving the camp without permission.