Tom Hansen, a hopeful greeting card writer with a firm belief in true love, meets Summer Finn, an enigmatic young woman who doesn't believe in relationships or boyfriends. Their meeting and subsequent interactions at the greeting card company where they both work set the stage for a complex romantic relationship. Tom is instantly smitten by Summer's beauty and free-spirited nature, seeing in her the potential for the kind of love he's always dreamed of. As they get to know each other, it becomes clear that their perspectives on love are markedly different, creating a romantic dynamic that's both whimsical and bittersweet.

The story unfolds in a non-linear fashion, zigzagging through various moments marked as specific days during the titular 500 days that define Tom and Summer's relationship. Through a collage of experiences shared between the two, ranging from the euphoria of new love to the depths of heartbreak, the film explores the complexities of emotions and expectations involved in modern romantic attachments. The narrative dives into Tom's memories and subjective experiences, illustrating how love can be simultaneously joyous and devastating as Tom grapples with his feelings for Summer, his own ideals of love, and the harsh realities of human connection.