"Param Sundari" follows Param Sachdev (Sidharth Malhotra), a spirited Delhiite who unexpectedly falls for Thekkepattu Sundari Damodaram Pillai (Janhvi Kapoor), a woman deeply rooted in Kerala tradition. Param, a somewhat spoiled rich kid, is looking to invest in a dating app and needs proof that it works. The app leads him to Sundari, and he lands at her homestay in Kerala to gather evidence for his investment.
Initially, Sundari isn't interested in Param and teams up with her uncle to teach him a lesson. However, as Param tries to win her over, he gradually falls in love with her. During the festive season of Onam, Param decides to confess his feelings, but Sundari is reunited with her childhood friend, Venu (Siddhartha Shankar), to whom she is betrothed. Sundari's Ammavan (uncle) announces that she will marry Venu to fulfill her parents’ wish. Venu and Sundari were childhood sweethearts but lost touch due to their careers.
Heartbroken, Param decides to participate in the annual Onam games. After being initially kicked off the team, he manages to get back in and ultimately helps them win the boat race. Param remembers Sundari's lesson to balance the boat by rowing in rhythm, and his idea of singing "Yeh Dil Deewana" motivates the team to victory.
As Param wins over the town, Sundari also develops feelings for him. However, when Param's father arrives and asks him to reveal the truth about his visit, Sundari is left heartbroken.
In the climax, following the boat race victory, Sundari is about to marry Venu. Param climbs a coconut tree to talk to her, and their conversation is overheard. Param is chased by the town residents and ends up at the land's end, diving into the water to escape. Sundari leaves Venu and rushes to rescue Param. Remembering a conversation about intimate moments, Sundari asks Param if he has kissed on a boat. When he says no, she kisses him in the middle of the water. The film concludes with the song "Bheegi Saree". Thus, the film ends with Param and Sundari together, suggesting a happy ending to their cross-cultural love story.
