Episode one examines the case of Mary Queen of the Scots. A curious cipher in her correspondences resulted in a charge of treason in 1586 for plotting to assassinate Elizabeth I.
Episode two profiles mid-19th century inventor and mathematician Charles Babbage, who was challenged to crack a particularly resilient code. This code is none other than the famous Vigenère cipher, which had been left unbreakable for 300 years.
The history of codes is examined. Included: the cracking of an encoded telegram from the German foreign minister to the president of Mexico during World War I.
Episode five examines public-key cryptosystems, which have been further developed into a cipher known as RSA (Rivest, Shamir and Leonard). Using prime numbers and algorithmic systems, RSA is used for secure data transmission across the globe.