SMG3 sunk past security to speak with The Imposter about a discussion they seemed to agree on, after the deal was made the next day he sets off on his mission he sees MM observing in his words"A beautiful day." He transforms into the shape of MM, knocks the real MM out, and infeltrates the castle.
Due to the fact that Smg3 only gave a picture of SMG4 to The Imposter, he does not know his target's name, thus making Mario suspicious, but he gives away SMG4's location, Starman3 asks Mario, where The Imposter (MM) is going, (being the snitch he is) Mario tells Starman3, as Starman3 intervenes The Imposter (MM) says he was just going to use the bathroom.
Thinking that Starman3 is on to him, The Imposter knocks out X and shapeshifts into him to keep his cover. Once he finds Smg4 he tries to kill him, but has trouble tring to keep up with him, once SMG4 has The Imposter (X) cornered he exited the room to show that The Imposter (X) was tring to kill him, but The Imposter shapeshifted into Mario naked.
At this moment Starman3 brings, Mario, SMG4, and Luigi up to speed on the day's current events, of The Imposter's 'performance' as the meeting is taking place. The Imposter shapeshifts from Toad back to his regular self and tries to shoot Mario, but he forgot to put ammo in his gun. At that moment his cover was officially blown, so he teleports to the basement to hide, then soon he is quickly discovered by SMG4 and rushes to Lethal Lava Land to face off with SMG4.
Once he is defeated by SMG4 he jumps onto a moving platform vowing that they will meet agian... before the platform falls and crushes him between it and the lava, burning him to a very hot temperature on impact with the lava and killing him, with SMG4, Mario and Starman3 watching the whole scene.
SMG3 is later seen at his grave outside the castle, calling him useless son of a b*tch.