Uthaman Pradeep is a 24-year-old support engineer working at Cognizant in Chennai. He lives with his widowed mother Saraswathi and elder sister Divya. He is in a romantic relationship with Nikitha, a software engineer who is the daughter of Venu Sastri, an orthodox Brahmin lawyer. They are a perfect couple, made for each other, and are ready to take their relationship to the next level i.e. marriage, but Pradeep hesitates a bit due to Nikitha's strict father. But Venu eventually finds out about his daughter's relationship with Pradeep and invites Pradeep over to his house, ostensibly to discuss about the marriage.

However, during their meeting, Venu toys around with Pradeep and then proposes a strange deal with him; he should exchange his mobile phone with that of Nikitha's for a day, along with the respective mobile's screen lock. After the end of the period, if they still feel that they are in love, Venu will not object to their union, else they must go their separate ways. Both Pradeep and Nikitha agree to this decision half-heartedly and exchange their phones, but Pradeep deletes his WhatsApp messages before doing so. Pradeep then returns home to manage Divya's upcoming marriage to Dr. Yogi, a dentist, which is to take place after four days.

On the goading of his friends, Pradeep goes through Nikitha's mobile phone, where he sees a WhatsApp conversation with Revi, Nikitha's boy best friend, who has a one-sided love for her and had unsuccessfully proposed her once. Pradeep is upset that Nikitha has a strong relationship with Revi, despite knowing about his one-sided love, and do many activities together. He further finds out that Nikitha is still friends with her ex-boyfriend Mamakutty and had gone on a night drive with him to Puducherry a few days earlier without his knowledge. He confronts Nikitha on both Revi and Mamakutty, leaving her in tears, and driving a wedge into their hitherto perfect relationship. The next day, Pradeep proposes to Venu that both he and Nikitha keep each other's phones until Divya's marriage is over, to know more about Nikitha's relationships, to which Venu agrees readily. However, unknown to Pradeep, Venu asks Nikitha to restore Pradeep's WhatsApp messages from backup to know about Pradeep's true colours.

Meanwhile, Divya and Yogi seem to get along very well, as Yogi, is a cultured and mild-mannered man, who treats Divya as an equal. But Yogi is secretive regarding his mobile phone and does not let anyone, including Divya, touch or even come near him while he is using his phone, which upsets Divya and causes her to turn suspicious about him. She, her friends, as well as Pradeep, unsuccessfully try multiple times to get Yogi's phone, open it and check its contents. This further contributes to the growing wedge between her and Yogi. Pradeep's and Nikitha's relationship worsens by the day, with Nikitha finding out about Pradeep's porn addiction as well as his conversations with multiple girls on WhatsApp, even managing to get them to send their photos based on the excuse of giving them an opportunity to act in a web series, but unknown to Nikitha, Pradeep has a second Instagram account on his phone, which he and his friends had created in college to message sexually explicit content using a fake female identity.

The same account was used to perversely message Nikitha as well as her younger sister Shwetha (though Pradeep did not send the messages). Pradeep realises this and meets Nikitha at a restaurant to try and convince her to hand over his phone for an OTP to remove the account from his phone. But Revi, who has accompanied Nikitha, discovers the account and exposes Pradeep. This leads to a big fight between Nikitha and Pradeep, which ends with them breaking up. Further, Pradeep beats up Revi for interfering in their relationship, only to be stopped by the restaurant staff. He later suspects that his close friend Mani is behind the messages and fights with him as well, despite his pleas that he did not send the messages.

During Divya's wedding reception, Pradeep notices both Divya and Yogi arguing over the latter's phone secrecy, and in anger, he tries to snatch Yogi's phone, only to be angrily stopped by Saraswathi. Pradeep breaks down and reveals to her about his breakup. Saraswathi consoles him and tells him that trust in relationship is all that is needed, which he realises when he finds the tree he had struggled to grow in his childhood. Later, while smoking with Yogi, Yogi reveals that the reason he hesitates to share his phone is due to others mocking him for his appearance. He feared that if he had shared his phone with Divya, she would have rejected him as well. Divya overhears their conversation and, contrary to Yogi's fears, respects Yogi even more for his confession and they both reconcile, with Divya refusing to look at Yogi's phone. Pradeep reconciles with Mani and his friends as well.

Meanwhile, a porn clip, apparently involving Nikitha, goes viral, causing Venu to assault and then disown her, despite her pleas that the video was doctored. Heartbroken, Nikitha leaves her house. Pradeep locates her sitting alone at a beach and consoles her. He reveals that he and his friends had managed to track the perpetrator of the video; Nikitha's publicity loving colleague Kaushik, who morphed Nikitha's face into a random porn video for the purpose of creating a trending video, and is arrested by the police. Pradeep and Nikitha reconcile and rekindle their relationship. Venu apologises to Nikitha for misunderstanding her and then reveals to her and Pradeep the real reason for the phone swap; he wanted to check how strong their relationship would be despite the hurdles, and informs that they had succeeded, thus approving their marriage. The movie ends with a message that trust is all that is needed in order to find peace in today's society.