This episode is a complete retelling of the tragedy that happened to the Hishigata siblings. The flashback showed everything: Mikihiko's research, their goals, why they were doing it and the exact moment when everything went terribly wrong for them. The pacing didn't feel rushed and I don't have any complaints about the animation since there wasn't any action in this episode.

P.S. If only the siblings had encountered Heaven Canceller then their fates would have been completely different