Deep under a mountain in Northwest Montana, two miners break into a cavern and discover an ancient space ship. Inside this ship is a space-time gate controlled by an extremely intelligent computer. This computer has self-awareness and can project any image it wants. It projects an image of a man and introduces itself to the miners as Joe.

It tells the miners it wants to hire them to enlarge the tunnel they made from the outside of the mountain to its cavern so it can get its spaceship out. The computer is in the shape of a small crystal. It tells the miners to detach it from the spaceship and take it with them through the space-time gate, and it will lead them to where it has thousands of tons of gold, stashed on a primitive planet. The gold would be their pay for enlarging the tunnel.

The men go through the gate and find themselves in a number of dangers that they hadn't bargained for, including a war.

How the ship got under the mountain and how the computer knew English is all explained in the book.