Eylül, Farah Zeynep Abdullah, is a young woman from a wealthy background who is currently dating a successful movie star, Onur Tuna. Despite being happy with her life, she admits that she does not feel truly in love. Her world begins to change after a car accident while traveling with her boyfriend, which leaves her with minor injuries but also causes her to forget the entire month of September—a period that holds significant meaning for her, as her name suggests.
Seeking answers, Eylül journeys to Bozcaada, a scenic island where she spent that very missing month. There, she encounters a man named Tekin, or Tek, played by Engin Akyürek, a gentle and reserved local. Tek, who is shy about his appearance and lives a quiet life, claims they have met before and that she had fallen in love with him. Curious and confused, Eylül pushes him to explain using his caricature drawings, which reveal memories she cannot access. The film alternates between her present-day conversations with Tek and the flashbacks of their relationship during that lost month.
In the past, Eylül was on vacation on Bozcaada Island with friends. Tek, who is a talented painter of caravans and signboards, falls instantly in love with her, despite his shy demeanor. His best friend, Serra Keskin, portrays Gülşah, a young girl who introduces Eylül and Tek. Initially, Eylül thinks of Tek as a fleeting summer attraction, especially as she is involved with her boyfriend Onur Tuna, whom she describes as her "success story" but not her true love. However, her boredom with her current relationship begins to grow, and she impulsively decides to extend her stay on the island, encouraged by Tek's persistent pursuit.
Tek invites her into his world through simple yet meaningful acts—shared breakfasts, watching sunsets, dancing at local gatherings, and convincing her to delay her departure. Despite Tek’s fears because he cannot swim, Eylül persuades him to join her in the water one day, deepening their bond. Over time, their connection intensifies, culminating in a night when Eylül's friend Berrak calls, but she doesn’t disclose that she’s with Tek; her feelings are becoming conflicted.
As they grow closer, Eylül and Tek share an intimate moment, falling in love. The next morning, Eylül wakes up feeling happy, but as she looks at her reflection, she faints in a panic, overwhelmed by how much she has changed and how deeply she has fallen for Tek. She calls Berrak to come and pick her up to leave the island. That night, while dancing with Berrak, Ceren Moray, and her boyfriend, Eylül sees Tek approaching and claims that Eylül is his girlfriend. She dismisses him, saying it was just a summer fling, and prepares to leave the island.
However, Tek appears one last time, desperately pleading for her to stay. Eylül laughs him off and departs, leaving Tek heartbroken. Unable to cope, Tek walks into the sea and drowns himself. This tragic event marks the moment when her forgotten September is finally restored.
In her current life, the realization floods back—the car crash was triggered when she saw an obituary for Tek in the newspaper. Panicked, she drove recklessly, desperately trying to find him. Back in the present, she dives into the water where she finds Tek's pen. Using it, she writes “I love you” in the sand, echoing the love he expressed for her earlier in the film. Heartbroken but at peace, Eylül leaves a note in Tek’s garden with his pen, creating a symbolic makeshift gravestone for him. Through this journey, she finally acknowledges her true feelings and the profound impact Tek had on her life.
