Mizuto and Yume used to date, but now they've suddenly become stepsiblings. Both of them agree that they don't want to do anything that'd disappoint their newly remarried parents. However, neither of them can accept the other as being the eldest sibling in the relationship. So they both agree to settle the matter with a game. Whoever does something in conflict with what people might consider normal sibling behavior, loses. Making them the younger sibling.
You may fancy yourself a
 hardcore mystery fan,
،لا بدّ أنّك خِلتني معجبًا كبيرًا بالغموض
Vielleicht hältst du mich
ja für einen totalen Mystery-Fan,
No sé qué pensarás tras leer
tantas novelas de misterio,
Creerás que soy un simple aficionado
a las novelas de misterio,
T’as l’air de te tromper
sur mon compte.
Forse mi hai scambiato
per un appassionato di gialli,
Deve estar pensando que leio
histórias de mistério demais,
Может, ты и сочла меня за
ярого читателя детективов,