(S01E20) This episode is listed as "String Theory" on my PVR and some places online, but on NBC's site and the screen itself it says "Five Years Gone" so that's what I went with.

"What if?" stories are fun. They allow the creators to play around with characters and show them in unexpected new lights. This isn't a purely "what if?" story, however. What happens here determines "our" Hiro's (younger Hiro's) ability to return to the present and change history in the coming episodes -- or fail to.