It turns out that Gregory was intentionally driving Paula crazy so that he could look for Paula's aunt's lost jewels in her attic without raising suspicion. Gregory was the murderer of Paula's aunt. Thanks to the detective work of the police, his scheme is exposed, Gregory is arrested and Paula is free from his sinister influence.
Note - the term "gaslighting" is derived and from and popularly coined by this film. The film is based on a 1938 play. Fearing competition from an earlier version of the film made in 1940, MGM attempted to destroy all copies and the negatives of the predecessor. They need not have worried as this film was far more popular and nominated for seven Oscars and won two. Today, the Gaslight DVD includes both film versions - this pooper is for the Oscar-nominated version.
The film begins with the young woman Paula Alquist (Ingrid Bergman) confessing her love for her debonair suitor Gregory Anton (Charles Boyer). The two decide to marry and they move to a house once owned by Paula's aunt (who bears a striking resemblance to Paula herself). Her aunt was a famous singer who was the victim of an unsolved murder. Paula is still deeply affected by her aunt's murder, so Gregory has all of the aunt's belongings moved up into the attic and boarded off from the rest of the house. Before all the goods can be removed, Paula finds a letter to her aunt from a mysterious lover named Sergis Bauer, who had planned to visit around the time of the murder. She tells Gregory, who takes the letter and tells Paula to ignore it as a random coincidence.
Their married life soon begins to crumble. Paula is seldom allowed to leave the house or accept guests. Her only companions are the housekeeper Elizabeth (Barbara Everest) and maid Nancy (Angela Lansbury), the latter of whom constantly flirts with Gregory. Paula becomes forgetful and also begins hearing strange noises and seeing the gaslights in the house dim while Gregory works at night. On one of her increasingly seldom trips outside, detective Brian Cameron (Joseph Cotten) notices Paula and recognizes her resemblance to her aunt, who was his favorite singer as a child.
Cameron decides to re-examine the aunt's homicide case and discovers that the aunt had several valuable jewels that were never found after the murder (nor were they ever sold). With the help of fellow police officers and Paula's nosy neighbor Miss Thwaits (Dame May Whittey), Cameron begins secretly monitoring the house. He discovers that Gregory disappears every night, only to arrive at his doorstep the next morning dirty and disheveled.
Meanwhile, Paula's fears that she is going crazy increase. In addition to hearing noises, losing things and seeing the gaslights flicker, Gregory accuses her of stealing items from the house, belittles her in front of Elizabeth and Nancy, and drives her to fits of hysteria in public. Gregory ultimately unveils that he discovered Paula's long-lost mother died in an insane asylum and that she had imagined the note regarding her aunt's lover.
Cameron eventually figures out that Gregory is actually Sergis Bauer. He deduces that Gregory was unable to locate the jewels after he killed Paula's aunt and thus married Paula so he could search the aunt's belongings unimpeded. Gregory is also driving Paula crazy (gaslighting her) so that he can abandon her in an asylum once he finds the jewels. That same night, Gregory (who had snuck into the attic each night through a sunroof, and whose searches had caused the sounds and gaslight flickers that Paula noticed) finds the jewels sewn into the aunt's opera gown. He returns home to have Paula committed, but is confronted and tied to a chair by Cameron.
Before the police arrive, Paula confronts Gregory. She acts as though she will free Gregory from his bonds and allow him to escape, but then mocks him by saying she is too "crazy" to actually help him out of his predicament. Gregory also admits that the desire to obtain the jewels were like a madness, a desirecould never drive from his brain.The police take Gregory away and the film ends with Cameron comforting a still-exhausted Paula on the roof of her house (suggesting a potential future romance).