The Mystery of the Chest is the fourtieth blooper created by SMG4.
The story begins with an black-covered figure sneaking into Peach's Castle, beating up Toad in the process. The intruder's revealed to be Mario, and subsequently presses a few buttons against a wall and enters a secret room, which contains a blue chest. Unfortunately, the chest's revealed to be empty, much to the red plumber's horror, causing him to bleep in anger, waking up several people in the process.
Mario then rounds up everybody at midnight, much to people’s annoyance. As he begins to interrogate everyone, he tried to intimidate everybody into speaking, which doesn’t work. Deciding to ask people what they remember from yesterday, he first asks Nintendofan997 what he did yesterday.
X’s perspective depicts yesterday as Mario giving him a bob-omb that the red plumber constructed yesterday. As the fuse is beginning to shorten, X desperately searches for a place to put the bomb. He decides on an empty room with Kirby sleeping.
SMG4 shares his point of view on how he heard the explosion coming from the room X set the bomb off with. Seeing a hole in the wall (as well as an unconscious Kirby), he explores into dark hole in the wall and sees a blue chest full of mushrooms. Shortly after deciding to eat one, he deems that the chest is useless, and discarded it off a cliff.
Luigi explains how he managed to find the chest that SMG4 discarded, and how he decided eat a mushroom while showing it off to King Bob-omb. The King, believing it to be a challenge for battle, charges (extremely slowly) forward, despite Luigi trying to tell him what he really wants to show. Luigi backs up into a corner, and after 2 hours, King Bob-omb finally picks up and throws Luigi out of his arena. Seeing the chest, and interpreting Luigi's words as to touch it (and of course, the green plumber telling otherwise), he ends up throwing it off a cliff, which ends Luigi's flashback.
Bowser recalls how a chest landed near him. Deciding to open it in hopes of drugs, he instead finds mushrooms. Settled, he ends up eating one and having a tea party with it. A bystander pokes fun of him, resulting in the Koopa King quickly throwing it off his base, much to his humiliation.
Toad tells how the chest crashed through Peach's ceiling in the castle. Deciding to eat one (in the irony of basically eating his own species), he then reluctantly gives the last mushroom to Lakitu. Peach, investigating what had happened, decided to toss the chest to the hole in the wall, essentially setting the entire plot into motion. Mario then enters.
Mario, still not figuring out the situation, then asks everybody who did it again. SMG4, fed up with Mario's inept investigation skills, tells the red plumber that he did it. Mario, buying SMG4's comment, then goes ahead and jumps off a cliff for his stupidity. The time then signifies 7 am, to which Kirby finally wakes up. The episode ends with the pink gumball asking if everybody had a good night's sleep, much to everyone's annoyance.