Katsuki stares back at Shoto for a few moments. Todoroki doesn't know that All For One is out of prison. That means he's probably from before the operation that went to shit, the operation where Dabi revealed the ugly truth about the Todoroki family to the world.

"Shigaraki broke All For One out of prison, and the world's going to end unless we successfully stop them. All of them." It's the most succinct answer. The most important bits.