Vaishnavi Trisha Krishnan is a popular YouTuber and chef living in Chennai, Tamil Nadu. Her friend Ambuja's fiancé Cotton is stuck in a situation that demands him to go to London to work as a chef, but he is unqualified and steals Vaishnavi's recipe to get a job. He asks Vaishnavi to accompany him to pose as an assistant and help him secure the job so that he can be financially stable. After Ambuja and Cotton's wedding, Vaishnavi agrees and travels to London with Cotton and a relative Balki Swaminathan. They stay with their friends Ganesh Ganeshkar and his wife Madhu Jangiri Madhumitha and work as chefs at a restaurant in the Park Grand Hotel in Hounslow. Vaishnavi meets Sandeep Jacky Bhagnani there, and they fall in love.
One day, on a boating trip in the River Thames, Madhu loses her bracelet but Vaishnavi finds it and a shankha comes along with it. Vaishnavi accidentally injures herself and washes off her blood in the water. This drop of blood triggers the vengeful spirit of a dead woman at the bottom of the river. She possesses Vaishnavi, who goes on a killing spree. Her friends experience numerous paranormal occurrences soon after and consult with a priest to find that Vaishnavi is possessed. The spirit turns out to be Mohini, a doppelgänger of Vaishnavi [Trisha Krishnan]. Meanwhile, Sandeep proposes to Vaishnavi, and they get engaged in the presence of his family. They meet Sandeep's mother Menaka [Poornima Bhagyaraj] and his estranged father Vicky/KVR [G. M. Kumar]. Mohini is enraged upon seeing him, and this reminds her of why she was killed. When Vicky/KVR announced Sandeep as heir to his Property, Mohini decides to kill Vicky/KVR and his goons who had killed her.
Mohini was an architect working for a construction company owned by KVR in London. She found out about the child sacrifice rituals (narabali) that KVR believes in and goes on to rescue children from a child trafficking gang owned by KVR. KVR and the gang are arrested, but they get released because of KVR's political influence. KVR and his men then capture Mohini, kill her, and throw her body into the Thames. Now that she has found a doppelgänger in Vaishnavi, Mohini returns for revenge. She goes after Vicky/KVR and kills him, but not before telling that all of his generation will be killed by her, and that includes Sandeep. Menaka, KVR's wife, learns of this and tries to protect Sandeep from Mohini with the help of a Buddhist monk Suresh. She also contacts Mohini's mother, who agrees to help with the rituals that they are performing. The monk tells that in addition to the rituals, either Vaishnavi's body must be destroyed or a human sacrifice has to be done to appease Mohini. Meanwhile, when Sandeep and Vaishnavi move into their new home, Vaishnavi sees a picture of Vicky/KVR whom Mohini had killed. This triggers Mohini's anger and tries to kill Sandeep and chases him into a church. Mohini taunts Sandeep that she will kill Vaishnavi. The monk calls Sandeep and tells him that there is a waterfall behind the church which makes the water holy. Sandeep tries to swim through the river to reach the monastery, where his mother, the priest, Cotton, Madhu, Ganesh and Balki are waiting, but Mohini cannot follow him through the river and can only run on dry land. When the two reach, They all prevent Mohini from attacking Sandeep and try to save Vaishnavi from dying in the process. Mohini's mother finds that because of her daughter's vengeance, two innocent lives will be lost. She realises that Mohini's spirit needs just one human life in order to be satisfied, so she kills herself and offers as a sacrifice to Mohini in front of the Goddess. Mohini's spirit is finally at peace and leaves Vaishnavi and Sandeep alone.
Later on, it is shown that Mohini's spirit still lives on and hunts down anyone who tries to cause harm to children, She possesses Neha, a child who was groped by the school bus driver.
