In the first season of the series the Moomin characters are getting into fights and causing general mischief. They also do things that weren't present in the original stories (like driving a car or even getting drunk at a bar). The Finnish author of the Moomin books, Tove Jansson, was very displeased of the darker and edgier tone of the series. She ordered that the original production company Tokyo Movie Shinsha (TMS) be removed from the show after the first 26 episodes.
The tone of the series changed more close to the original series under the new company Mushi Production (which did the remaining 39 episodes from the first season). Mushi also produced 52 more episodes for the second season.
