Plot

Titu is going to marry a perfect woman named Sweety. Titu's best-friend Sonu doubts Sweety's character and tries to break the marriage while Sweety tries to do opposite and which leads to war between Bromance and Romance.

We have always been told that "Marriage" is a marriage between two families. 'Sonu ke Titu ki Sweety' explores the new age version of the same belief where "Marriage" is a marriage between two friend circles. What happens when you decide to get married, but your best friend and your girlfriend can't see eye to eye. Sonu and Titu are childhood friends. Titu has always been a naïve lover, who falls in love easily and Sonu has always had to step in to save him. But Sweety is the perfect bride and Titu is head over heels in love with her. Sonu's instinct to protect Titu at whatever cost, makes him feel that she's too good to be true. What ensues is a war between the best friend and the bride to be. While Sweety is trying to impress Titu and the family alike, Sonu is out to sabotage the utopian romance.

Titu (Sunny Singh) is crying because his girlfriend Pihu (Ishita Raj Sharma) has stopped talking to him. Sonu (Kartik Aaryan), his best friend tells him that she is not the right one for him as he trusts her with everything, even the keys for the cash counter of his sweet shop, but Pihu wants to break up simply because he opened Tinder on her phone which she should not even have in the first place. Sonu says that Titu has been in a relationship with Pihu for 2 years and has not been happy after their first month anniversary. She has constantly tried to change Titu, as she doesn't like him wearing traditional clothes, eating sweets, watching Punjabi TV serials, changing their toothpaste to herbal, introduces Titu as her "friend", dances with other guys at parties and has convinced him that all of this is somehow Titu's fault. Her Instagram feed does not have a single photo of Titu. Sonu gives an ultimatum to Titu to choose between Pihu and him and Titu chooses his best friend.

Titu lives with his family father Dharam (Pawan Chopra), mother Manju (Ayesha Raza Mishra), grandfather Ghasita and grandmother Rukmani (Madhumalti Kapoor). Lalu was a worker at their sweets shop called Ghasitaaz, who gradually became Rukamni's brother and Ghasita's drinking buddy and now owns a minority stake of 7% in the sweets shop. Ghasita constantly razzes on Dharam for a drop in quality of his sweets ever since Ghasita retired. Meanwhile Sonu runs a marriage event planning service. Sonu and Titu have been friends since nursery. Sonu also lives with the family ever since his mother passed away. Now he calls Manju his mother. But since Sonu's father is alive, he doesn't call Dharam as his father. Sonu's father is now settled in Canada, but he stayed behind to be with Titu.

Six months after his break-up, Titu's family tells him that a family (father Parag (Rajesh Jais) and mother Renuka (Deepika Amin)) has shown interest in getting him married to their daughter. He agrees to meet Sweety (Nushrat Bharucha). Sonu says that Titu does not need to get married to fulfill his "needs".

Titu and Sweety two instantly hit it off, Sweety is extremely caring, sensitive and is the 'ideal wife'. Titu is very trusting and tells Sweety about the power dynamics of the house in the very first meeting. He says that Daadi controls the cash in the house and even his mother hasn't ever got the keys to the house from Daadi. And that he had a breakup just 6 months ago and that Sonu advised him. The family likes Sweety, but Sonu wants more time to decide. Sonu finds this very fishy, though he admits to Titu that she seems nice, but he can't really say whether or not the two should marry just yet. Under pressure from the family to make a decision, the boys agree to a Roka ceremony. Sweety runs an NGO for kids. Sonu believes that she is too good to be true. Sonu shares his concerns with Ghasita and Lalu. Ghasita refuses to intervene as he believes that he did his job by raising them and giving them a good education and business. Sweety decides to organize a "Maata Ka Jagraata" at Titu's place.

Sonu investigates the matchmaking agency that matched Titu and Sweety's profiles. Through them he meets Sweety's ex and finds out that Sweety was in fact going to marry another man named Rahul but that did not work out as she became dominating, possessive, and got drunk and created a scene in Goa. Sonu reveals this to the family and Sweety accepts everything. Titu tells Sonu that she had told him all of this in the first meeting itself and he did not want the family to know about all of this. Sweety tells Sonu that the guy had a girlfriend whom he never broke up with. He demanded a dowry as well. And then he wanted to have sex with her before marriage, and when she resisted, he started all these rumors against her. Sweety tells Sonu that she considers him as her brother and that he should have checked with her before going to the family. On the night of the engagement when Sonu begins warming up to the idea of Titu and Sweety's wedding Sweety reveals to him that she in fact is clever schemer and challenges him to reveal it to Titu and his family.

Sonu confides in Ghasita (Alok Nath) and Lalu (Virendra Saxena) about her revelation. And they bet among themselves to see if Sonu can save Titu from Sweety. One day Rukmani finds a matchbox in Lalu's hands and suspects that Ghasita is smoking. Lalu has to swear on her Rakhi to get her to believe that Ghasita is not smoking. Later, Lalu deals with Ghasita for a 4% additional stake in the company for the lie he spoke to Rukmani. Titu is now engaged to Sweety, and this brings a lot of compromises in Sonu's life who was earlier used to living a bachelor life. Sweety changes their caretaker with her loyalist Babu (Pritam Jaiswal), she walks in on Sonu making out with a girl (Sonalli Sehgall). Babu does everything that Sweety tells him to, and this interferes with Sonu's food, clothes, and all other aspects of his personal life. Sonu tackles Babu by harassing him endlessly by getting him to do even all the small tasks around the house, like handing him the remote control. Then, he lures Babu into watching porn, as Sweety walks in with Titu. She has no option but to fire him.

Sweety also manipulates the family members and Titu into buying and naming a house after her and Titu. The family goes to see a house, and Sonu expects Sweety to demand a registry under her and Titu's name. But Sweety asks for the house to be registered under Daadi and Manju's name. Daadi instructs Lalu to register the house under Sweety and Titu, and to make sure that Sweety does not have to go to the registrar office for formalities. Sweety gets the keys to the house from Daadi, and this comes as a complete shock to Sonu and Lalu. Sweety turns the house into vegetarian (by taking Daadi to the butcher shop) and threatens Ghasita of revealing his dirty lies to Daadi.

Sweety then lures Titu into having sex with her. She then threatens Sonu that she will get Titu jailed for sexual assault, if Sonu does not fall in line. Meanwhile Titu feels embarrassed as he thinks that he forced himself upon her and is now even more determined to marry her.

Sonu foresees Titu's downfall and to protect him he plans a bachelor party for Titu in Amsterdam where he arranges his ex-girlfriend Pihu to "accidentally" run into them. Sonu had convinced her that Titu is still in love with her, and he wants to help them get back together. Sonu tells Ghasita that he has used Pihu before and thrown her out of Titu's life and the biggest point that she never challenged Sonu in front of Titu. They return to India along with Pihu (Sonu invites Pihu to the wedding), and Sonu is happy to see a bothered Sweety after witnessing the growing closeness between Titu and Pihu. But Sweety reveals the truth to Pihu that it was in fact Sonu that broke Titu and Pihu up in the first place and that Sonu is using her. At this point Titu is visibly angry with Sonu.

He once again confides with Ghasita and Lalu. Ghasita volunteers to sort the situation, but Sonu says his best friend should decide for himself. Sonu helps prepare for the wedding, dances in the Baraat but during the garland ceremony he makes a final attempt and gives Titu the ultimatum again: 'It's either her or me'. Sweety attempts to turn the tide by stating some logical statements. Titu, in a fit of epiphany, reveals to Sweety that this is barely the second time he has ever seen Sonu cry, the first being three days after his mother's death when they were thirteen; his tears show what Titu means to him, and Titu firmly announces that if it is between Sonu or her, it will always be his best friend. The scene switches to Sweety standing in the Mandap and coming to terms that her malicious tactics cannot always work. After the narrow escape, Sonu, Titu, Ghasita and Lalu are seen sitting and drinking by the pool realizing that good girls don't exist these days.