As a child, Mike Capoferri accidentally killed his brother, shaping him. Doctor Stuart told him that time was a "one-way street." After Mike moves to the moon, he proves the doctor wrong.
The moons' new human resident discover alien artifacts on the moon--one a ship, inside a mysterious pyramid. Mike fiddles with the pyramid by instinct and finds himself in another time.
He guesses he's some three billion years in the past as the Earth is nearer than anticipated. He wants to prove he was there, without time paradoxes. He carves a hole with a sculpting pencil, which he accidentally lets go of. When he tries to leave again, the moon is disintegrated and he has to go back further in time to save the moon. After a few more mistakes, he travels back to find the aliens themselves,
Not just a cool onion-concept of time--revising and revisiting ideas of time as a street--Mike's past comes back to haunt his "present" as he discovers a third way of seeing time--a problematic one.