Following the death of Ludwig van Beethoven, a mysterious letter is discovered among his possessions. Addressed to an unnamed "Immortal Beloved," the letter sets off a compelling quest to discover the identity of this secret lover. Beethoven's executor, Anton Schindler, embarks on a journey to find the recipient of the letter, delving into the composer's past relationships. Schindler's investigation reveals a complex web of romances and friendships that Beethoven had throughout his life, providing insights into the emotional world of the renowned yet troubled composer.

As Schindler traces Beethoven's history, he encounters several women who were close to the composer, including his long-suffering sister-in-law and a few other pivotal figures in Beethoven's life. Each woman brings Schindler closer to unraveling the mystery, as he learns more about Beethoven's struggles with love, loss, and his deteriorating hearing. This journey not only illuminates Beethoven's romantic connections but also presents a portrait of a man driven by passion, both in his music and personal affairs. Schindler's quest becomes a journey of understanding, painting a picture of Beethoven as a figure tormented by his inability to capture the same beauty in his relationships as he could in his music.