From the introduction:
Epic Level Handbook talks about several ways DMs handle the existence (or non-existence) of epic-level characters in their world. While the possibility exists that epic-level characters have always been around, this is unsatisfying. An alternate suggestion is that PCs need to encounter some sort of Power or Event that allows them to go beyond level 20. This is such a story.
The story assumes that the PCs will want to return home to their world of origin. It is not necessary that they do so, although this adventure would be somewhat pointless if they stay. There are a number of ways to handle this. First of all, there are likely to be reasons the PCs want to return: love, family, duty. Even though the people of this realm say that the Heroes have earned their rest, it is usually not in the nature of Heroes to stay put for too long. Perhaps there is unfinished business back home, or there is yet another crisis facing their homeworld. The other alternative is to simply end the story early and let the Heroes retire. Not every player wants to take their characters into Epic levels, after all.