Vault of the Beast — A.E. Van Vogt

It came to Earth to find a genius mathematician . . . the one human in the solar system capable of understanding the ultimate prime number. Time is running out. The time-lock on the Beast's millennia-old Martian cage is about to open, and mankind will never be able to deal with the threat!

A classic from the Golden Age of science fiction, originally published in the August, 1940 issue of Astounding Science Fiction.