It is a beautiful warm August day in 1880: perfect weather for the annual Metropolitan Police Annual Fête held at Alexandra Palace. Inspector Best is summoned to uncover the identity of 'Quicksilver' who has sent an anonymous note threatening to cause an horrific explosion at the event. When a second note is received and its threats become increasingly confusing with their literary allusions, Best seeks out the help of Helen Franks, a close friend from the past. However, is Quicksilver really intent on causing mass injury on this fine day, or is his desire of a more personal nature?