This was originally going to be a short straight-to-video project, but Toshio Suzuki was so impressed with Hiroyuki Morita's storyboard, and especially the main character of "Haru", that he encouraged Hayao Miyazaki to give this cinematic release.
The trivia item below may give away important plot points.
The English-language title of this movie is "The Cat Returns". There are a number of interpretations of this title. The character of the Baron is recurrent: he originally appeared in Mimi wo sumaseba (1995) as a statue, but was nonetheless one of the main plot-points of the film. The title could also be a reference to the notorious fish-thief Muta alias Renaldo Moon who has returned to the cat kingdom after many years (and he also originally appeared in Whisper of the Heart). Another possible explanation is the return of the cat prince at the end of the feature to set everything right. Another possibility is Yuki, the "dirty kitten" that Haru helped as a child, and "returns" to save Haru as Haru has saved her.