The story follows Lee Do-Hyun who plays the role of Oh Ha-Rin, a woman who lost her first love, Ji Seok, to suicide on her birthday 10 years ago. Ha-Rin still carries the pain of his loss and has never been able to move on. However, her life takes a mysterious turn when she starts receiving old photos of her and Ji Seok from the past. Each photo triggers memories of their time together and makes her question if Ji Seok's death was really a suicide or if there was something more to it.
As Ha-Rin delves deeper into the mystery of the photos, she discovers strange connections between Ji Seok's death and a powerful family who may have played a role in his demise. Along the way, she meets a mysterious man named Shin Woo-Yeon, played by Jinyoung, who seems to have a connection to both her past and present. As Ha-Rin unravels the truth behind Ji Seok's death, she also starts to develop feelings for Shin Woo-Yeon, leading to a complex love triangle between the three characters.
Through flashbacks to the past and present-day scenes, Blue Birthday explores themes of love, loss, redemption, and the power of memories. As Ha-Rin unravels the truth behind Ji Seok's death, she also learns to heal from her past trauma and open her heart to new possibilities. Ultimately, the drama delves into the idea that true love never dies, even in the face of tragedy and mystery.