It’s been some weeks since Ranko and Astrid solved the mystery aboard the Marine Express submarine train during the special trip organized by St. Joachim Academy, the school they attend. Their lives have returned to normal, but that doesn’t mean there won’t be more mysteries to solve in the future. The day before the school’s big Christmas festival, the two friends are finishing their second term tests, but despite this being a crucial time for the alumni, the girls find themselves drawn into solving a new enigma.A first-year student, Veronica Bell, approaches them to ask for their help investigating the disappearance of the sheet music for the big choir concert, which is scheduled as the final event before the school holidays. She suspects that one of the rebellious singers, known as “the Seven Bad Apples” of the academy, has hidden the scores in order to cancel the concert and avoid having to perform. Veronica needs those back before the performance the following day, so they decide to help her find