Saravanan, a psychiatrist, visits India on vacation. He meets up with his foster brother Senthilnathan alias Senthil, and his wife Ganga. Senthil's mother Kasthuri wanted Senthil to marry Priya, the daughter of his father's cousin Kandaswamy, to reunite the two branches of the family after 30 years of separation because Senthil's father chose to marry Kasthuri instead of Kandaswamy's sister, Akhilandeshwari. Saravanan learns that Senthil had bought the Vettaiyapuram mansion, despite attempts by the local village elders to dissuade them, and moves in with them. Akhilandeshwari is jealous of Saravanan and plots to kill him with the help of her assistant Oomaiyan.

When the family visits their ancestral temple, the chief priest reveals the reason everyone fears the Vettaiyapuram mansion. A hundred-and-fifty years ago, a king named Vettaiyan travelled to Vijayanagaram in Andhra Pradesh, where he met and fell in love with a dancer named Chandramukhi. However, she did not reciprocate his feelings as she was already in love with a dancer named Gunashekaran. As a result, Vettaiyan took her back to his palace by force. Unknown to him, Chandramukhi made Gunashekaran stay in a house nearby and had meetings with him secretly. When Vettaiyan discovered this, he beheaded Gunashekaran on Durgashtami and burnt Chandramukhi alive. As a result, Chandramukhi's ghost tried to take revenge on Vettaiyan, who with the help of various priests and sorcerers from all over the country, tames the ghost by locking it up in a room located in the palace's south-west corner. Priya is in love with Vishwanathan, a dance professor who reciprocates her feelings. Their love is supported by Saravanan, who requests Kandaswamy to arrange their marriage.

After hearing Chandramukhi's story, Ganga, who thinks that the story was fabricated to scare thieves from stealing treasures in the room, wishes to go there. She gets the room key from the gardener's granddaughter Durga and opens the door to the room. Saravanan sees a conversation between Durga and Ganga, and runs away, thinking that Durga is possessed, because a flash of light appeared when both of them were speaking. Subsequently, strange things begin to happen in the household; a ghost frightens the people in the house, things inexplicably break, and Ganga's sari catches fire. Suspicion turns towards Durga. Senthil immediately calls Saravanan to solve the case. As soon as Saravanan returns, a mysterious being tries to kill Priya. Attempts to kill Senthil are made with poisoning his coffee and by pushing a fish tank on top of him. A mysterious voice sings during the night. Saravanan investigates these incidents.

Ganga mysteriously disappears during Priya and Viswanathan's wedding reception. Saravanan notices her absence and searches for her, but he is almost killed by Oomaiyan—who has been sent by Akhilandeshwari. Saravanan subdues Oomaiyan and with Senthil's help finds Ganga, who is supposedly being sexually harassed by Viswanathan. Saravanan reveals to Senthil and Viswanathan that Ganga suffers from split personality disorder. Saravanan explains to them how she became affected by it and how she took up Chandramukhi's identity. He tells them Ganga tried to kill Priya and Senthil, and framed Viswanathan for sexual harassment because from Chandramukhi's view, Viswanathan is her lover Gunashekaran since he stays at the same place Gunashekaran did. By framing Vishwanathan, Chandramukhi wanted to stop the reception. The only way to stop Chandramukhi is to make her believe Vettaiyan is dead since Saravanan impersonated Vettaiyan and disrupted one of the pujas conducted by the exorcist Ramachandra Acharya by conversing with the ghost to know its wish.

Akhilandeshwari overhears Saravanan's idea of self-sacrifice and apologises to him. Later, in the dance hall, the family and Ramachandra Acharya allow Chandramukhi to burn Saravanan alive. Ramachandra Acharya blows smoke and ash on Ganga's face when she is given a torch to burn Saravanan. Senthil then opens a trapdoor to let Saravanan escape, and an effigy of Vettaiyan gets burnt instead. Convinced that Vettaiyan is dead, Chandramukhi leaves Ganga's body, curing her. The two families are reunited after 30 years; Saravanan and Durga fall in love, and the couple Swarna and Murugesan become parents after eight years of marriage.

Director P. Vasu, producer Ramkumar Ganesan and Raj Bahaddur make guest appearances in the song "Devuda Devuda".

Development

Chandramukhi was the 50th film produced by Sivaji Productions. During the success meet of Mannan (1992), Rajinikanth announced that he would act in Sivaji Productions' 50th film. In September 2004, Rajinikanth congratulated P. Vasu on the success of his Kannada film Apthamitra (2004) and was impressed with the film's screenplay. Vasu narrated a story of the film to Rajinikanth. Rajinikanth later called Ramkumar Ganesan and asked him to do the film in Tamil under Ramkumar's home production banner, Sivaji Productions.

Ramkumar telephoned Vasu, who was offering worship in a temple in Guruvayoor at that time, informing him of Rajinikanth's wish to do Apthamitra in Tamil under his direction. Vasu reworked the script he wrote for Apthamitra to suit Rajinikanth's style of acting. The film dealt mainly with the concept of dissociative identity disorder, commonly known as "multiple personality disorder" (MPD) or "split personality syndrome". Another film titled Anniyan (2005), which featured Vikram in the lead role, and was released two months after Chandramukhi, was also based on the same disorder.

Thota Tharani was the film's art director, and also designed the costumes used in the film. Regarding the designing of the Vettaiyapuram palace, he watched both Manichitrathazhu and Apthamitra to get the basic idea of the film's plot. Manichitrathazhu was shot in a palace in Kochi. Vasu wanted Tharani to make the sets more colourful and grand and did not want the realistic look of the original film. Tharani designed Chandramukhi's room and placed a corridor in it, which was quite different from the original and its Kannada remake. The corridor resembled those commonly seen in palaces in Kerala. Sidney Sladen did additional costume designing in the film.

Casting

Rajinikanth played the roles of Dr. Saravanan and King Vettaiyan. He sported a wig for his role. Both Rajinikanth and P. Vasu discussed every scene featuring the former and added necessary inputs before they were shot. Rajinikanth appears in the beginning of the film unlike Manichitrathazhu, where the same character, played by Mohanlal, appears in the middle of the film. Rajinikanth requested Vasu to change the dancer's name from Nagavalli, which was the name of the danseuse in Apthamithra, to Chandramukhi since the latter sounded more royal. While Vishnuvardhan uttered "Haula Haula" in the original, Rajinikanth used "Lakka Lakka", which was based on the mannerisms of a villain in a Marathi play Rajinikanth watched in his childhood days. Ramkumar's brother Prabhu played Senthilnathan, a civil engineer and owner of Ganesh Constructions. Prabhu co-produced the film with Ramkumar.

For the roles of Ganga and Chandramukhi, Soundarya was initially selected to reprise her role from Apthamithra, but her death led the director to choose Simran and he shaped the character to suit her. In November 2004, Simran refused to do the project, as her role required a lot of dancing and cited her pregnancy at that time as another main reason for her refusal. Aishwarya Rai Bachchan was offered the role but she declined the offer due to schedule conflicts. Sadha and Reemma Sen were also considered as replacements. The role finally went to Jyothika, who gave 50 days of her schedule for the film. Vasu wanted Jyothika to perform her scenes in a manner different from that of Shobana's role in Manichitrathazhu, enacting the scenes himself before they were shot featuring Jyothika. Nayantara was selected to portray Rajinikanth's love interest, Durga, after Vasu was impressed with her performance in her debut film Manassinakkare (2003).

Vadivelu portrayed the comic role of Murugesan, Akhilandeswari's and Kandaswamy's younger brother. Rajinikanth, at the film's 200th day theatrical run celebration function, said that it was he who recommended Vadivelu to Vasu for the role and had asked Ramkumar Ganesan to get Vadivelu's dates before planning the filming schedules. Nassar played Kandaswamy, Murugesan's older brother. Sheela, who was known for her role in the Malayalam film, Chemmeen (1965), played Akhilandeswari, the intimidating older sister of Kandaswamy and Murugesan. When Sheela was signed on for the role, she was requested by the p