Professor Gordon Naismith unexpectedly discovers that he’s a warrior Shefth from the future, and now the Uglies from the future wants him to return to kill an alien Zug who managed to get through the time barrier that’s meant to keep out the Zugs.

The full version, called Beyond the Barrier, was published shortly after the shortened two-part serial (about 45,000 words) appeared in F&SF.
Let us say there was a need to be inconspicuous. This is a dead period, for hundreds of years on either side. No one knows about this abandoned liner except us, and no one would think of looking here.

Variants

(1)
  1. The Tree of Time by Damon Knight, Magazine of Fantasy and Science Fiction, Dec 1963 to Jan 1964.
    2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . written by Damon Knight