Midsummer Tempest — Poul Anderson

Suppose every event in history had at least two causes and led to and from a separate universe in space and time.

There would be an infinity of alternate worlds.

Imagine a world in which every word Shakespeare was literally true. A world in which Prince Rupert of the Rhine could fight to save Charles I of England and escape the Puritans by hopping a railroad locomotive, on the right track but 200 years before its time. A world in which an aging Caliban could pace the lonely shore, yearning for the return of his lost love, Miranda.

And where all these universes, and more, intersected, there lay a tavern where infinite possibilities met--if only for a night!