My Maîtresse was written on intention to expand the storyline of Love Live! from previous works of the series, being the 5th book and counting. The story arch continues the relationship between Eli Ayase and Umi Sonoda, involving those who are close to them that may have affected the main characters to go down the road they are now.
The story is written in 7 chapters, just like The Girl who Sails the Seven Seas, by Eli alone. The storyline of this book takes place throughout time from the beginning to the most recent, across other books as well.
The purpose of this book is to show the hidden secrets of Eli and how she came to being the girl Umi loves, to also explore the big argument that leads the main characters to distant as suggested in the series.