Prime numbers are undeniably one of the most beautiful and fascinating topics in mathematics. But what are prime numbers? Are they only numbers that are divisible by one and themselves, or do they have another interesting hidden face? Throughout history, the mystery of prime numbers has challenged the greatest minds in mathematics starting from Euclid of Alexandria to Fermat, Euler, Gauss, and Erdős. All of them attempted to solve the puzzling problem of primes. The achievements they realized and the secrets they revealed can only affirm how deep prime numbers are.

Starting from how prime numbers exist in nature, and how they are of great use in modern cryptography on which our daily life completely depends, the author travels in the holy kingdom of primes diving into some conjectures involving those special numbers. From the Riemann Hypothesis and the well-known zeta function, he explains how a note in the margin turned to be Fermat's Last Theorem, one of the most important problems in the history of mathematics. From Mersenne Primes, he gets to the twin primes, those shining little stars in the dark sky of primes. And from Euclid's proof of the infinite number of primes, he gets to a hidden pattern in the distribution of primes discovered by Stanislaw Ulam and called the Ulam Spiral. After this little trip, you will know, dear reader, why prime numbers deserve to be called "the holy grail of mathematics".