Chapter 4: After days of living in relative comfort, Mario is forced from his cell by Kamek and a group of Koopatrols. They inject him with a thick, green liquid before roughly escorting him out of the dungeons (to his first trial [chapter cuts off prior to]).
Chapter 5: Luigi navigates an eerie ghost house in search of his brother. After some sadistic hi-jinx (including more than a few illusions), the younger Mario brother is captured by King Boo.
Chapter 6: Princess Peach retires for the night after a tiresome meeting with her royal advisors regarding the disappearance of the Mario brothers. With both heroes missing, and castle guards diminished due to dozens of search parties, the princess is quietly kidnapped from her chambers.
Chapter 7: Mario awakens in his cell to find Luigi (in a separate cell) and Princess Peach (w/ Bowser). They watch as Mario is again forced from his prison, injected with green goo, and escorted out of the dungeons, shouting as he leaves (insistent that the Princess and his brother not be involved in the torture [trials]). Bowser brings the Princess with him to watch the trial while Luigi is left to let his fears fester.
Chapter 8: Post-trial, Peach in hysterics over the death of Mario, only for Bowser to reveal he's still alive. Peach begs Bowser to put a stop to this twisted game, even offering her hand in marriage, but he refuses.
Chapter 9: Trials have become a regular occurrence, with Mario a constant participant. Luigi, while also still trapped in the dungeons, never participates in trials, and is doomed to witness his bloody, beaten, broken older brother return from them each day. Prior to entering the trial, dialogue reveals that Luigi serves as an understudy of sorts to Mario, should the older brother ever refuse to participate, but Mario has (and always will) enter the trials instead of his younger sibling.