“By His Bootstraps,” Robert A. Heinlein, 1941 – Portal-based
This short story by one of the great masters of science fiction was originally published in the October 1941 issue of Astounding Science Fiction under the pen name Anson MacDonald. In the story, Bob is finishing up his thesis on — what else — time travel, when he is interrupted by a man named Joe, who declares that he has come from the future via a ‘Time Gate’ and asks him to come through. Before he can, another man appears, and a fight ensues. Needless to say, if you’re at all familiar with this genre, but all are versions of Bob from one point in time or another, and the story continues with Bob navigating his way through the time tangle, going back and forth through the gate that leads thirty thousand years in the future.