The film revolves around Vasanth (Prashanth), a stock analyst, and his wife Malini (Meena). The two are in search for a flat in Chennai. Vassanth finds the perfect place on the 12th floor of a high-rise apartment building. However, the apartment has a horrifying past. The previous occupant of the flat, a young woman Manju who had killed her child and jumped from the balcony and died. Malini learns about this incident shortly after moving in and becomes oddly fixated with the story. Then, a series of inexplicable experiences drive Malini to near madness.
Vasanth becomes helpless and convinced his wife has developed some sort of psychological disorder. He consults a psychiatrist, Dr. Ranjan (Sarath Babu) after his treatment fails Vasanth begins to doubt that his wife is suffering from a psychological disease. The couple’s maid (Kalairani) believes Malini to be possessed, and calls in an exorcist (Suhasini).
Meanwhile, other seemingly unrelated events take place around the building. The watchman was gruesomely murdered with his head completely twisted, one of the residents Ajay (Abbas) is behaving erratically, and a murder occurs. The Inspector (Thiagarajan) is sent to investigate the murder at the building. The Exorcist spends time with Malini and learns that Malini is in fact possessed by the Manju. For more help, Vasanth goes to the Manju's mother (K. R. Vijaya). With the mother's help, the possessed Malini lets everyone know that Ajay lusted after Manju and forced himself on her, and to save herself she jumped from her balcony. Later, Ajay realised that her son had witnessed the murder, and got the watchman to throw the little boy from the balcony, and creating a fake story that the woman had killed her son and then committed suicide.
By killing the watchman via Malini, the Manju takes revenge for her son's death and now wants to kill Ajay. At the last minute, the mother comes and tells her dead daughter's soul that she cannot take revenge from Ajay by making Malini a murderer, so the woman leaves Malini's body and Ajay is lifted above ground and thrown about in front of everyone, including Dr. Ranjan and the inspector (who first ridiculed the story). The story ends with Ajay going to prison and the inspector telling him how people like Ajay make him sick and tells him to rot in a jail cell forever. After the inspector leaves the darkened cell, Ajay finds himself face to face with Manju. He starts begging for mercy, but his voice soon fades out as Manju draws closer; it is implied that she kills Ajay.Prashanth as VasanthMeena as Malini/ManjuAbbas as AjayThiagarajan as Paramasivan (Police Inspector)Sarath Babu as Dr. RajanSuhasini as an exorcistDevan as a psychiatristK. R. Vijaya as Manju's MotherKalairani as the maid
In October 2003, director Thiagarajan bought the Tamil remake rights of Ram Gopal Varma's 2003 Hindi supernatural thriller Bhoot in October 2003, after the original had become a box office success. Thiagarajan's son Prashanth and Simran were signed on to play the lead roles, though the actress later opted out citing her impending marriage as a reason. The team then announced that actress Reemma Sen had replaced Simran in the film, though Sen called the statement "premature". Reports then suggested that Laila had replaced her though this proved to be untrue. The leading female role was later taken by Meena, who lost weight to portray the role which she described as her "most challenging" til date. An early press release had announced that Asin, Kushboo, Ramya Krishnan, Raghuvaran and Pasupathy would also be in the film, but none of the actors eventually starred. Similarly Kanika and Sujatha were initially speculated to appear in the film, but did not make the final cast. Abbas joined the cast to play a negative role, while K. R. Vijaya, Suhasini and Sarath Babu were also selected to essay supporting roles. Subtle changes were made to the script of the Tamil version to make it adaptable for Tamil audiences. During production, the team revealed that they hoped to introduce a new form of technology in the film which would prevent the film being seen on unlicensed copies. The film's shoot was completed within twenty six days, with meticulous pre-planning arranged by Thiagarajan. Thee makers also considered producing a Kannada and Malayalam version of the film, but eventually did not carry through with the idea.
The film opened in July 2004 to positive reviews, with a reviewer from Indiaglitz.com noting the makers "deserve an appreciation for his honest and sincere attempt on the screen." Malathi Rangarajan of The Hindu wrote "Even at the outset director Thiagarajan has to be congratulated upon for not hampering the tempo with the usual frills of solos and song and dance routines", though added " the film doesn't frighten you as much as they said it would". Sify.com's reviewer credited Meena's performance noting "watching her work up a maniacal frenzy or slip into a pathetic state of helplessness, only to let out a deathly scream, is an experience not worth missing". Another critic also noted the director "has done a wonderful job in creating a thriller with minimum dialogues", adding "Meena carries the whole film on her shoulders and has given a splendid feat. Meena’s mind-blowing act that actually saves Shock from its rickety script." Nowrunning.com noted "comparisons with the original might work against "Shock", but it is worth a scare". The film became a surprise success at the box office.