かつてふしぎ星の危機を救ったおひさまの国のふたご姫、ファインとレインももう15歳。『ふしぎ星始まって以来のプリンセスらしくないプリンセス』と言われたのももう昔の話…と思いきや、今日も二人はドタバタとお城の中を走り回っています。というのも、２人の発案から始まった久し振りのプリンセスパーティがおひさまの国でもうすぐ催されることになったからです。各々成長し忙しくなっていく中、中々集まることが難しくなっていた仲間達ともうすぐみんな会えるかもしれない。２人が楽しみじゃないはずがありません。そしてもう一つ楽しみなのは招待状を一人一人に配りに行くこと。直接顔を見て元気な姿を確認したいのです。「たいへんたいへん！早くしないと間に合わなくなっちゃうよ〜」「だいじょうぶだいじょうぶ。まだ日にちはあるわ」リトル・プリンセス号に飛び乗って６つの国へいざ出発！果たしてふたご姫は全員に会え、無事パーティに出席してもらうことは出来るのでしょうか？ちょっぴり大人になったふたご姫の新たな冒険の始まりです！ ………なんてね。

Fine and Rein, the Twin Princesses of the Sunny Kingdom who once saved the Wonder Planet from crisis, are now 15 years old. It seems like a long time ago that they were called "the least princess-like princesses since the beginning of the Wonder Planet"... but today, the two are still running around the castle. The Princess Party that was started by the two of them is going to be held in the Sunny Kingdom for the first time in a long time. As they grow up and get busy, it has become difficult to meet up with their friends. It's impossible for them not to look forward to it. And another thing they are looking forward to is handing out invitations to each person. They want to see their faces in person and make sure they are doing well. "Oh dear! If we don't hurry, we won't make it in time~" "Don't worry, don't worry. We still have time." Hop on the Little Princess and set off to the six kingdoms! Will the Twin Princesses be able to gather everyone and have them attend the party? This is the beginning of a new adventure for the slightly more grown-up Twin Princesses!…Just kidding.