"All You Zombies..."

A young man walks into a bar and meets a time-traveling bartender whose origins -- and relation to the young man -- are more convoluted and stranger than the snake-swallowing-his-tail ring on the barkeep's finger.

Robert A. Heinlein's "All You Zombies—" is considered one of the most influential and thought-provoking short stories ever to tackle the mind-bending nature and paradoxes of time travel.