Immensee is primarily about loss—the loss of youthful love and innocence. It is a recurring theme in Theodor Storm's work, and it appears in a very similar form in his later novella Hans and Heinz Kirch, which also features a young man and woman separated and reunited after a long period only to find that what they had before cannot be rekindled.

In Immensee, Reinhard and Elisabeth have an idyllic existence as children. He reads fairy tales to her, and a bond is established between them. Later, as a student and throughout his entire life, Reinhard looks back wistfully on this period. The atmosphere of childhood is symbolized by Christmas, the sweets his mother sends him when he's a student, and the fairy-tale atmosphere of the holiday. None of it lasts, of course. Elisabeth marries another man, a friend of Reinhard. When Reinhard visits them and is alone with Elisabeth, he asks her what has become of their youth. No answer is given: the story's basic idea is that the dreams and intentions of youth can't be recaptured.

Immensee is similar to another even more famous work of German literature, Goethe's The Sorrows of Young Werther. Both concern a man who fixates upon an ideal love that is beyond his reach, except that in Werther the passions depicted are much more desperate and lead to the young man's suicide. Storm's version of this theme is gentler, and Reinhard survives, though unable forever to capture the joy he once had when young.