The has been unsolvable cases the Scotland Yard wasn't able to solve. Queen Victoria decided to intrust the most intelligent criminal to solve these crimes, and in return, she gets her freedom and a reduced life sentence after each case she solves. The criminal Queen Victoria chose, was Kim Taeyeon. Number 1 in the most wanted list. The one rule Queen Victoria gave to Taeyeon was, whoever she chooses to be in her team has to be approved by the Queen. 

Before Kronos was sent down to Tartarus, he cursed the 6 main gods that put him in this predicament. Kronos' curse amplified their flaws and caused them to be unworthy as gods. They had to be reborn as humans with the same flaws personified as monsters chasing them until they can overcome their weaknesses to go back to their godly status.