Ek Din follows the story of Dinesh 'Dino' Srivastava (Junaid Khan), a shy and introvert IT employee in Noida who secretly loves his colleague Meera (Sai Pallavi). Unnoticed and overlooked, Dino watches helplessly as Meera continues a relationship with her married boss, Nakul (Kunal Kapoor). During a company trip to Japan, Meera discovers a painful truth about Nakul and is left heartbroken. She stays back alone, while Dino quietly remains nearby out of concern. An accident leaves Meera with temporary memory loss, she forgets the present day entirely. Mistakenly assumed to be her partner, Dino pretends to be Nakul to keep her calm. As they explore Japan together, Dino finally experiences a fleeting chance at love.