Jerry Welbach, a reluctant bagman, has been given two ultimatums: The first is from his mob boss to travel to Mexico and retrieve a priceless antique pistol, known as "the Mexican"... or suffer the consequences. The second is from his girlfriend Samantha to end his association with the mob. Jerry figures alive and in trouble with Samantha is better than the more permanent alternative, so he heads south of the border.
A fleet of Martian spacecraft surrounds the world's major cities and all of humanity waits to see if the extraterrestrial visitors have, as they claim, "come in peace." U.S. President James Dale receives assurance from science professor Donald Kessler that the Martians' mission is a friendly one. But when a peaceful exchange ends in the total annihilation of the U.S. Congress, military men call for a full-scale nuclear retaliation.
