Leroux's original 1910 novel is set in late 19th-century Paris at the Opéra Garnier, which was built between 1857 and 1874. During construction it became necessary to pump underground water from the foundation pit of the building, creating a huge subterranean river. The employees claim that the opera house is haunted by a mysterious ghost who wreaks chaos and destruction when displeased. Joseph Buquet, the lead scene shifter of the opera house continually tells stories of how he has seen the "Opera Ghost" (which was partially a trick played on him by other opera house workers) to the ballet girls who are very frightened of this "so called" ghost who haunts the opera. Joseph Buquet goes on later to say that "The ghost had Death's head!", and changes his story to say that "The ghost had a head of fire, but with no body attached to it!", but later described the body as being "a dress suit that covered a skeleton", and later in the chapter "Is it the Ghost?", Joseph Buquet dies a mysterious death, which the opera ballet girls immediately blame on the opera ghost. "The Opera Ghost," is actually a horribly deformed musical genius named Erik. He uses the "Ghost" facade to send the managers of the Opera Garnier repeated threats of catastrophe should they not pay him a monthly salary of 20,000 francs and perpetually reserve Box Five for him at every show. This arrangement, unbroken during the many years of the managers' tenure, is abruptly terminated when two new proprietors, Armand Moncharmin and Firmin Richard, take over the opera house and refuse to give in to what they view as empty threats, thinking that it is a practical joke by former managers.

Meanwhile, Erik has taken on a protégé, Christine Daaé. He explains to her that he is the "Angel of Music", a heavenly spirit sent by her dead father to help her, and proceeds to give her regular voice lessons through the wall of her remote dressing room. Under the tutelage of her new teacher, Christine makes rapid progress in her vocal studies and mysteriously achieves sudden prominence on stage when she is selected to replace Carlotta, who was suddenly ill that day. Christine stuns the audience with her seemingly new vocal talent when she performs selections from Faust. During the performance she faints on stage, which deeply troubles her good childhood friend Raoul, Vicomte de Chagny.

Erik becomes envious of Christine's relationship with Raoul and finally appears to her in person, wearing a mask to hide his features. He takes her to his dark world beneath the opera house. Christine quickly finds that there is nothing angelic about Erik; she learns with disappointment that he is "neither an angel nor a genius", only that he and the ghost are one and the same - just a man. She comes to know him as malicious, volatile, dangerous and somewhat bitter, yet also brilliant and pitiful. She is infuriated at having been deceived and demands to be set free. Erik promises to release her after five days. After some awkward moments (dining by herself while he watches, being shown his room which looks like a death chamber, his bed a coffin), Erik and Christine eventually begin a duet from Othello, and Christine rips off his mask, dying of curiosity. "If I live to be one hundred, I should always hear that superhuman cry of grief and rage which he uttered before that terrible sight reached my eyes", Christine later tells Raoul. Erik is furious at having his deformity exposed to someone who, he thinks, could love him. He threatens to keep her in his home forever, but later changes his mind. Christine is released, but only after promising to return by her own will and swearing never to give her love to anyone else. Christine does return out of pity and fear.

But Erik isn't the only one with an envious nature. After Christine's debut performance, Raoul overhears her succumbing to a tyrannical, yet supernaturally beautiful, disembodied voice in her dressing room (Erik). He becomes suspicious that another man is taking advantage of her innocent belief in an "Angel of Music" in order to seduce her. He starts spying on her in an attempt to find the mysterious seducer. Christine suddenly becomes aware of this and is very angry but, after Erik reveals himself to be the Ghost (and after Raoul's threat of leaving Paris on a suicidal polar expedition), she decides to tell Raoul, on the roof of the Opera Garnier, everything that has happened between her and Erik. The two of them plan to run away from Paris and the "horror of Erik".

Erik eavesdrops on their conversation, and comes up with another one of his ingenious plans of action. He abducts Christine from the stage during her final performance at the Opera Garnier as Marguerite in Gounod's Faust, at the point where Christine, as Marguerite, is appealing to the angels to carry her soul to heaven (the aria is best known as "Anges purs, anges radieux"). Raoul follows them down into the depths of the cavern beneath the opera house, and is guided to Erik's house by a character known as the Persian. Unfortunately for both of them, the route they take to Erik's house leads instead to a torture chamber (a catoptric cistula), where they are captured by Erik. Raoul and the Persian listen helplessly as Erik rages at Christine, accusing her of lying to him and betraying him. He threatens that should Christine not marry him, he will destroy the Opera Garnier. Christine, already on the brink of suicide, sadly accepts his proposal at 11pm the next night, Erik's "deadline".

Eventually, Christine shows Erik genuine sympathy and displays her love for him by being kind to him despite his extreme ugliness, not running away when he takes off his mask, and even kisses him on the forehead. This grants Erik a happiness he never thought possible. In despair, Erik releases Raoul and Christine and gives them his blessings to marry. He asks only that Christine come back after his death, and bury him with the ring he gave her. Erik dies three weeks after he lets Christine and Raoul go.

Right before his death, Erik delivers a dramatic monologue expressing his grief, in which he describes how Christine was the only woman to let him kiss her, his brief euphoria when she kissed him, his despair at having the love of his life betrothed to another, and his gratitude to the Persian daroga (Persian for "chief of police"), who once saved his life.

The English word 'emotion' is derived from the French émotion and émouvoir. This is based on the Latin emovere, where e- (variant of ex-) means 'out' and movere means 'move'.^[1] The related term "motivation" is also derived from movere.

Researchers distinguish feeling and emotion, where feeling refers to the subjective experience of the emotion. Some believe that emotions can occur unconsciously, and hence that emotion is a more general phenomenon than its subjective feeling. Feelings may also more narrowly refer to the experience of bodily changes. A second distinction focuses on the difference between the emotion and the cause of the emotion. For example do we say that thoughts about a loved one cause the emotion of love or that these thoughts are part of the emotion? One way to resolve this issue is to see whether the emotion can occur independently of these thoughts. Thus, thoughts about a particular person or situation could not be part of the emotion of love, since one can experience the same emotion about many other things.

Yet could one experience love without some thought or other of a loved person or object? If not, then we may stipulate that thoughts of a loved object are part of the emotion. Some theorists argue that at least some emotions can be caused without any thoughts or indeed 'cognitive activity' at all. They point to very immediate reactions (e.g. LeDoux 1996), as well as the conjectured emotions of infants and animals as justification here. Debate on this point is ongoing but represents a major distinction between what are called 'cognitive' theories of emotions and 'non-cognitive' theories of emotions, where non-cognitive theories regard some other feature of emotions, such as bodily responses to be essential.

A related distinction is between the emotion and the results of the emotion, principally behaviours and emotional expressions. People often behave in certain ways as a direct result of their emotional state, such as crying, fighting or fleeing. Yet again, if one can have the emotion without the corresponding behaviour then we may consider the behaviour not to be essential to the emotion. However some theorists such as Nico Frijda who hold a functionalist approach to emotions point to the idea that emotions have evolved for a particular function, such as to keep the subject safe. If the behaviours associated with an emotion are the determining factor for the very existence of that emotion then goal-directed behaviour should be regarded as essential to the emotion. Yet since we recognise that the behaviour need not necessarily occur, we can stipulate that emotions involve what are called 'action tendencies'. So for instance, fear involves the tendency to flee, which means that the probability that the subject will flee from a given situation is increased when he is undergoing fear