Main Story[]
At first, Frisk for some reason doesn't fall underground, causing someone else to fall: Boyfriend. The human in this case, is him, who was possessed by Chara after falling into the undeground running away from his girlfriend killer.
Though Chara managed to possess Boyfriend, they seem to have stuck with his style of settling things with rap battles, keeping his mic to lash out when needed. Leaving the ruins, the two are faced with Sans, stranger than usual, wondering why the human is different this time. Chara seems to freak out when they see the skeleton with the same determination as them, and goes on the attack. Sans smiles, and they have a match that is a cross between singing and a real battle, with one part imitating the Last Corridor, and Girlfriend's ghost appearing to support one side. Sans, despite being determined and persevering, recognizes that he will eventually die in vain, so he decides to flee after the end of the battle.
While in Snowdin Town, Papyrus worriedly calls out his missing brother's name, wondering where he might have gone. Boyfriend arrives at the scene and Papyrus notices his presence, asking him if he knows where his brother is and saying that everyone in town has disappeared. Papyrus about to leave the place, realizes the current situation again and decides to capture Boyfriend for being a human, so he accepts the silent rapper's singing game. Seemingly amused, Papyrus laughs before being hit by a blaster fired by Sans, who mocks the human and tells him to "be faster next time". Papyrus tries to resist death with his head, but ends up giving in and falls down to his own brother.
Homie-cide[]
In Dustswap universe, the story is the same as Dusttale, but US Papyrus takes the place of Sans, and US Frisk takes the place of Chara. At Muffet's restaurant, US Sans asks his brother what happened to the place, completely devastated and without lights. The two sing, but Papyrus does not back down and remorselessly kills his brother, who in the afterlife is completely mentally destroyed by what happened.
Notice: This section mostly follows the dialogue and animations, not the opening cutscene, as that seemingly deviates from what happens in-game.
The Murder - Anthropophobia[]
Soon after he begins, he encounters Boyfriend. Confused by the sudden and unplanned arrival of a human, he panics and teleports deeper into the Underground, to Waterfall. But to his surprise, Boyfriend is hot on his trail. Despite not having a significant amount of power yet, he is forced to battle against this human whom he doesn't recognize.
After finishing a rap battle, Boyfriend senses a foreign presence behind him, sending chills down his spine. Before he can act, his body is possessed and he is forced to attack Sans, dealing 20 damage to him. Retreating, Sans teleports away as a confused and scared Boyfriend regains control over his body. Girlfriend curiously stares at Boyfriend, as if questioning what had just happened.
Some time later, Sans is seen attacking Undyne, the head of the Royal Guard. After being blasted into a cave, she stumbles out, her body melting as it struggles to resist death through sheer determination. Turning around, Sans confronts Boyfriend and Girlfriend, who were observing from close by. Having no choice but to fight, Sans engages Boyfriend in another rap battle, leaving Undyne to melt, as Girlfriend... takes a seat on her shoulder.
As Sans and Boyfriend finish another rap battle, Sans, seemingly talking to himself (really to Phantom Papyrus), insists he's doing what he has to do, and that he doesn't know what he's going to do after he's done. Not knowing who Sans may be talking to, a confused Boyfriend initiates another rap battle.
Sans sends a couple of bones towards Boyfriend, but before he can get hit, his body is once again possessed, and he finishes off Undyne with a knife. Sans teleports away, having failed to gain any LV or EXP from this encounter.
Eventually, Boyfriend strolls into the Judgement Hall, spotting an idle Sans. Alerting Sans, Boyfriend challenges Sans for yet another song. An enigmatic, yet familiar figure stands behind Boyfriend, watching, watching, waiting for the right opportunity. Sans' pupils light aflame as the final battle begins. Two rap battles ensue, with Sans mixing in increasing amounts of attacks into his verses. But even after all that, Boyfriend is still standing confidently. Sans looks down, almost amused, before Boyfriend feels that something is... off.
Boyfriend freezes as his eyes flash a crimson red, static filling his mind. Then he attacks Sans, manifesting a knife out of thin air. Boyfriend's mouth gapes to an inhuman level and his eyes are replaced with a vantablack void, a viscous liquid oozing out of his face. Caught off-guard, Sans' left pupil pulses an intense violet as he prepares for one last rap battle, this time putting more of his backbone into it.
Genocide Ending[]
Despite his best efforts, Sans was defeated, his HP being slowly whittled down as Boyfriend sent slash after slash at him. Exhausted, Sans' eyes begin to close as his attacks cease, the familiar melody of his lifeless hometown quietly playing in his head. Boyfriend eagerly slashes at Sans, but he dodges once more, forcing his heavy body to sidestep another attack. Then Boyfriend suddenly appears behind him, the skeleton's eyes widening as his life is suddenly taken from him.
Satisfied with his efforts, Boyfriend smiles with content, but this is short lived. Another skeleton appears behind him, seemingly forcing itself into existence. This one is different; it possesses no body, or arms. It's just a floating skull and a pair of hands, wrapped with a blood-red scarf. A familiar grin sits on its face. It's Papyrus, Sans' late brother, come to avenge his late brother. He begins singing, and Boyfriend obliges. Reality itself seems to shake and dissolve as they battle it out, and it becomes difficult to find what sounds to utter, but Boyfriend forges on nonetheless, the "same" co*cky grin on his face. After a brief rap battle, Papyrus seems to fade away as the hallucinations end.
Boyfriend smiles, knife in hand.
Pacifist Ending[]
Even with the blade in his hand, the bloodlust in his eyes, the visible insanity, Boyfriend has yet to attack Sans once more. Not to mention, Boyfriend himself is still alive and well. A perplexed and frustrated Sans stares down at Boyfriend as he suddenly halts, his expression rapidly swapping between that of struggle and genocidal lunacy. But deep inside Boyfriend's mind, he's fighting something, the very source of these impulses:
Chara, The First Fallen Human.
Blade in hand, Chara smiles wickedly at Boyfriend. There's one way to get this thing out of his head, and it's precisely what he does best: Rap. Microphone in hand, he sings as Chara strikes him again and again with their knife. Boyfriend does not falter, even if he gets terrifyingly close to being taken down then and there. And soon... he is able to force them out of his head.
Sans is waiting for Boyfriend back in reality, his vengeful pupils seemingly absent and with it, his insanity as well. He smiles at Boyfriend, not with a smile of pain nor murderous intent, but... just his usual big, dumb skeleton smile. Yes, the funni skeleton man-child has returned.
"see ya' in the next reset, kid"
