The hero must cross the threshold between the world that is familiar and that which is not. This often involves a threshold guardian that works to keep all with in the protective confines of the world, but must be encountered to continue on the journey and enter the new zone of experience. Sometimes, the hero crosses over in a way dubbed "belly of the whale." When this happens, the hero, instead of crossing a threshold, will pass into the new zone via rebirth. The hero, appearing to have died by being swallowed (hence, belly of the whale) or by having his flesh scattered, is transformed and becomes ready for the adventure and trials that he now faces.
Peter's crossing:
Peter crosses the threshold when he explodes over Kirby Plaza. His crossing is, in a way, a combination of encountering a threshold guardian and rebirth. Nathan has fought against the idea that the world is in danger from a supernatural force up until now. But he assists Peter to cross by taking him up into the sky, saving the world. Yet Peter still dies by explosion, his flesh scattered. He has crossed his threshold and will now face his trials.
Hiro's crossing:
After Hiro learns how to do what he must do in order to save the world, he arrives in Kirby Plaza. This time he is able to kill Sylar, but Sylar remains alive long enough to send Hiro shooting through the air. Quickly, to save his life, Hiro teleports. But he teleports to 1671 Kyoto. He has crossed the threshold from the familiar to the unknown.
Peter and Hiro both cross the threshold, but they do it in very different ways. Peter crosses via rebirth, Hiro crosses into the past. Both of them will face their trials, both of them in unfamiliar territory. Their real adventures begin now.