That's from the short Back to the Future crossover, whose title I can't remember. Here it is, it's called Back To Back To .... A sentence that didn't describe the overall plot might have been more of a stumper, but if you like you can demand a drabble on the basis that I couldn't remember the title. Also, I don't think I'd've remembered that was on the index, but there's so little on that index with Peri in it (as opposed to the list of stories I've circulated privately).