One moment, Aurin is just a Riolu enjoying a moment of peace in the woods. Next, he is a Lucario waking up in someone's house with no memory of how he got there or how he evolved.
He discovers he is one of sixteen Pokémon kidnapped under similar conditions, and everyone, Aurin included, is shiny. Aurin fears for the worst: Had a shiny hunting trainer gotten a hold of them?
The truth, he learns, is far worse.
Aurin and co. learn that they have been trapped, and the only way of escape is to get away with murder. Should they kill one of their fellow shiny Pokémon without getting discovered, then and ONLY then do they get the right to escape from the killing game. Meanwhile, if the remaining Pokémon fail to catch the culprit, they face execution.
There's nothing Aurin wants to do less than play the killing game, but he seems to have little choice in alternatives. As time lengthens with no clues and enticing motives poured on, how can these sixteen shinies avoid the temptation of murder to finally get the freedom they've been seeking?