Twenty-one year-old Kizie Basu is fighting thyroid cancer when she meets twenty-three year-old Immanuel "Manny" Rajkumar Junior, who has previously suffered from osteosarcoma and is in remission. Manny and his friend Jagdish "JP" Pandey, who is suffering from glaucoma and is blind in one eye, are making a film together in which Manny plays the male lead, inspired by well-known Tamil actor Rajinikanth. Manny invites Kizie to be the female lead. The two bond over his love for Rajnikanth's films and her love for Hindi music, specifically an incomplete song by retired songwriter Abhimanyu Veer. Kizie and Manny gradually fall in love as they shoot scenes for Manny and JP's film. They decide to make 'Seri', the Tamil word for 'okay', their secret word that will help them remember that everything will be okay in life and that it is important to stay positive. After an operation, JP loses sight in his second eye, making him go blind.
One day, Manny informs Kizie that he has managed to track down Abhimanyu and get in touch with him. Kizie e-mails Abhimanyu, who replies that he'll answer all her questions if she's able to visit him in Paris, where he lives. Kizie and Manny convince Kizie's parents to allow them to take the trip and manage to convince them, on the condition that Kizie's mother joins them. As they are making arrangements for the trip, Kizie's cancer worsens and she is suddenly hospitalised.
After her recovery, she is weakened and initially distances herself from Manny, but they eventually reconnect. They go to Paris to visit Abhimanyu, who turns out to have no conclusive answers and no reason for not finishing the song, disappointing Kizie. Soon after, Manny informs Kizie that his cancer has returned and that he is now terminal.
As Manny's health deteriorates, Kizie convinces him and JP to finish the film. Manny then invites JP and Kizie to his mock funeral, where they deliver eulogies that they have both prepared. Manny dies a couple of days later, leaving a letter for Kizie, explaining that he had finished Abhimanyu's song for her and had even convinced Abhimanyu to help him with the music despite considering him a terrible person.
JP's finished film premieres in an open-air theatre to a heartwarming response from the crowd, which consists of Kizie and Manny's friends and family. In the final scene of his film, Manny breaks the fourth wall and directly speaks to Kizie, telling her to live life to the fullest, to which she replies "Seri".