Mathilda, the wool mouse, lives unnoticed under the sofa in Anna's apartment. When a thunderstorm blows Mathilda's sister onto the windowsill and into the rain, where she dies, Mathilda is heartbroken. Crying, she draws Anna's attention to herself. Together, they give Mathilda's sister a dignified burial under a tulip in the garden, and this experience brings them together as best friends.
They then spend a cozy evening together doing crafts. Mathilda discovers books, meets the family dog "Wickie," and ponders strange job titles. However, when she gets into a heated argument with Anna while cleaning and vacuuming, Mathilda decides to run away…