A villainous organization kidnapped the souls of the main characters to forge links and steal the worlds' fortune.
To make a world's consciousness sever the link of its own accord and protect itself, someone has to impersonate the kidnapped character and fool it into believing that the soul fluctuations coming through the link are fake.
That's Yun Yan's job: act a part and deceive a senile, bereaved victim whose child has gone missing (the world's consciousness: ?), while his colleagues rescue the lost souls.
But there are too many lost souls, and the Samsara Soul Domain doesn't have enough people, so everyone has to take on several roles.
Then the worlds of Bungo Stray Dogs, Bungo Stray Dogs IF, The King's Avatar, Grave Robbers' Chronicles, and Detective Conan were greeted by an enormous screen that blotted out the sky. They watched the “protagonists” of their worlds take part in a competition that would decide whether their worlds survived.
When the competition entered its second round, everyone was shocked to discover that the worlds the contestants were sent to were all parallel versions of their own.
In the Naruto world, teaming up with Hashirama and Madara just after the village was founded, they faced off against Madara, who had left the village and was practically a god, and successfully convinced him to stay;
In the Bungo Stray Dogs IF timeline, what should have been a battle between two teams was thrown into chaos by the Port Mafia and the Armed Detective Agency, who also kept fooling a wanderer who was, in essence, a white-haired Dazai;
In the Grave Robbers' Chronicles world, when the Bronze Gate opened, one of the six people who walked out was actually Wu Xie? Meanwhile, an archaeological team elsewhere unearthed someone with dragon horns;
In The King's Avatar world, they won their challenge match by default because their opponent overslept. As soon as they stepped outside, a six-eyed man dressed in a neon samurai outfit chased them with a sword. After Ye Xiu was saved by his alternate self, he began a fantastical daily life full of one crisis after another;
The Bungo Stray Dogs world...
[Content warning]
I'm doing my best to keep the characters in character, but my writing and brainpower are limited, and my memory is terrible. I'll do my best to fix any plot holes. My deepest apologies!
Theme: Even in the most desperate times, people are still striving to save others