Arms smuggling, a missing person, government secrets. Three unlikely young people are drawn into a web of mystery and deceit when they set up their own detective agency, Williams, Mathers and Phelps. When papers go missing from a Whitehall safe, all three keyholders are beyond suspicion. Where is the fourth key that can unlock the mystery. The trail leads the young detectives across oceans to foreign shores, but sometimes secrets are hidden much closer to home.