In a gentle, whimsical world populated entirely by animals, a small mouse named Nezumi-kun receives a wonderful gift from his mother: a hand-knit red vest. He loves the vest dearly and is very proud of it. However, as he shows it off, one by one, his larger animal friends ask if they can try it on. Wanting to be polite and friendly, Nezumi-kun agrees. A duck puts on the vest, stretching it a bit. Then a monkey pulls it on, stretching it even more. Finally, a large elephant tries to squeeze into the tiny garment, causing it to become enormously stretched out and completely misshapen. Nezumi-kun is devastated as he looks at his ruined gift. In the end, the original vest is transformed into something new and different, like a swing, which allows Nezumi-kun to enjoy a new kind of fun with the very friends who accidentally ruined his treasure. This teaches him a gentle lesson about sharing, friendship, and finding happiness even when things don't go as planned.

Another major narrative arc in the series focuses on a female mouse named Nemi-chan. She decides to make a new red vest for Nezumi-kun. Nezumi-kun becomes excited, dreaming about the wonderful present and imagining all the things it could be. However, Nemi-chan struggles with the wrapping, and when she finally presents the vest, Nezumi-kun must learn to be strong and overcome his natural weakness for Nemi-chan, leading to more humorous and heartwarming situations as he tries to protect the new vest from the same fate as the first. Their relationship is a key part of the series, as they celebrate birthdays together and share adventures, with other animal friends like a monkey and a bird always nearby to join in the fun or help with a problem.

The series is an adaptation of a classic series of Japanese picture books that began in 1974, brought to life with modern CG animation that retains the simple, soft charm of the original illustrations. Each episode presents a self-contained, lighthearted story centered on the little mouse's interactions with his friends in their colorful, storybook-like setting. The humor comes from the contrast between Nezumi-kun's small size and big feelings and the larger, sometimes clumsier animals around him, while the warmth comes from the consistent themes of kindness, cooperation, and the gentle resolution of small childhood dilemmas.