Ramachandran "Ram" Krishnamoorthy, portrayed by Vijay Sethupathi, is a professional travel photographer who finds himself nostalgic during a visit to his former high school in Thanjavur. His feelings of reminiscence are intensified when he is added to his classmates' WhatsApp group, setting in motion the plans for a school reunion in Chennai. Excitedly attending the gathering, Ram reunites with close friends, including Bagavathi Perumal as Murali, Devadarshini as Subhashini, and Aadukalam Murugadoss as Sathish.
During the reunion, Murali brings news that Janaki "Jaanu" Devi Saravanan—Ram's childhood sweetheart, played by Trisha Krishnan—is coming from Singapore to join the event. Reminiscing about their days as tenth-grade classmates in 1994, the film vividly depicts the mutual romantic feelings that blossomed between the talented singer Jaanu and Ram. However, complications arose when, following their final board exams, Ram's family faced severe financial hardship, forcing them to relocate to Madras before the new academic year. Jaanu returns to find Ram's seat empty and ultimately moves on, believing they are forever separated.
At the reunion, Jaanu scans the crowd for Ram, and when their eyes meet, a flood of long-buried emotions surfaces. Their initial conversation is marked by awkwardness, yet as they step away from the commotion of the gathering, they find comfort in each other’s company. They share updates about their lives, including Jaanu's marriage in Singapore and Ram’s decision to remain single. As the night unfolds, an emotional late-night drive through Chennai allows Jaanu to reveal that she has never truly forgotten him. She expresses remorse over her circumstances, which led to her marrying someone else.
As their conversation deepens, Ram discloses several missed opportunities that leave Jaanu in shock. He recounts a visit to her college, where, unbeknownst to her, he had hoped to see her; however, she dismissed a message he sent, thinking it was from a stalker. Furthermore, Ram admits that he attended her wedding in secret, standing at the back of the hall, and was an invisible witness to the beginning of her married life with another man. This revelation tears at Jaanu, who had felt a mysterious connection with him on her wedding day, secretly longing for him to stop the ceremony. Facing the reality of their separation, Jaanu asks Ram to spend the remaining hours before her flight back to Singapore together.
Their night continues as they roam the streets of Chennai, encountering a group of Ram's photography students who mistakenly believe that Jaanu is his wife. To navigate the awkwardness, Jaanu concocts a heartwarming tale of how they reunited after college and have remained a couple ever since, with Ram going along with her spirited narrative. As the rain pours down, Ram suggests they seek refuge in his apartment, providing her with dry clothes and a shelter from the storm.
While at his apartment, Jaanu is touched to find an array of childhood mementos Ram has kept over the years—including her old school uniform, dried flowers, and handwritten poetry. Confronted with the depth of Ram's unwavering affection, she gently encourages him to move beyond his past, marry, and start a new life. In an emotionally poignant moment, Jaanu honors a promise from their school days by singing "Yamunai Aatrile," a song Ram had longed to hear her perform live for decades.
As dawn breaks, Ram drives Jaanu to her hotel to collect her luggage and then to the airport. Their parting is filled with a bittersweet embrace, as Jaanu prepares to embark on her journey back to Singapore. Once home, a reflective Ram carefully folds the dry clothes Jaanu had left behind and tucks them into his suitcase of memories, thus sealing away the cherished remnants of his first love forever.
