Both parties figure out what they have to do.
Kaneki’s new form, induced by stress, is a direct result of ingesting the artificial Quinxes. It turned him into a rampaging centipede that stomped all over Tokyo. It had no target, freely attacking both allies and enemies, for its only purpose seems to be to destroy. It has since settled down, but with the knowledge that it will reactivate again.
This is no longer a threat towards only the CCG, or only the ghouls, but something that will threaten them both. To that, the ghouls realize that they can’t handle this catastrophe alone, as they don’t know where to start. Although one of the clowns tries to suggest that perhaps this form is for the best, Tsukishima disagrees, saying that he wants Kaneki to be happy as a person, not just as a symbol, reuniting all the ghouls.
In a wildly different move, the ghouls confront the CCG, this time asking for cooperation. They have been natural enemies, but in the face of a bigger threat, this all falls away. They know they can’t stop the coming disaster alone, but the CCG is still reluctant. How could they agree, after all that happened? However, the incident is more important than their preconceptions and former antagonism, and they agree to join sides in the face of a disaster.
Mutsuki also reappears, this time distraught over the fact that his actions caused Kaneki to go into this new form. Furuta teases him of such, and he can’t say anything to defend himself. Back with the Quinxes that were his former comrades, he attacks them in the ho5pes that he can atone with his own death. He feels that in killing himself in battle, he will be able to apologize for jumpstarting this mess in the first place (never mind the fact that it wasn’t completely his fault). Urie and Yonebayashi quickly figure out what he’s doing, and don’t fight their fullest against him- after all, a former comrade is suffering. Even when Mutsuki ends up trying to kill himself, Yonebayashi quickly stops him. She tearfully points out that they’re still friends and comrades, and after all this time she still loves Mutsuki. The same can be said of Urie and the other Quinxes, and Mutsuki breaks down, realizing that what he wanted- acceptance- was something that he had all along. Even if the world is falling apart, people are still sticking together.