On board the Dutch liner bound for Southampton Sir Lionel Barton has bag articles of insignificant value in the wrappings of the Mocana relics and has turned them over to the purser to be placed in the ships safe. During the night the safe is robbed by agents of Fu Manchu who with the supposed Mocana relics leap in to the sea and are picked up by a sea plane. Upon his arrival in London with the authentic relics intact sir Lionel Barton and his niece Rima go on immediately to Barton's Norfolk estate while Sean Grevile and Paul Rickson, a friend of Grevile's take up their quarters in Barton's town house. As the two friends arrive home after a late dinner the voice of Fa Lo Swee, Foo Manchu's daughter calls to Grevile from a basement window of a neigboring house. A they enter the house to investigate they are overpowered by several silent and unseen figures...