When the Imperial State Crown is stolen from the Tower of London, the Home Office assigns Captain Francis Blake to assist Chief Inspector Glenn Kendall of Scotland Yard.
Blake in turn calls in his old friend and housemate, Professor Philip Mortimer, who has been on holiday to Scotland but agrees to return to London to help in the enquiry.
Meeting Blake at the Centaur Club, Mortimer is also introduced to some of its regulars: Leslie Macomber, editor of the Daily Mail; Sir Hugh Calvin, judge at the Central Criminal Court; Professor Robert Vernay of the British Medical Association; and Dr Jonathan Septimus of the Psychiatric Institute.
This is to no avail: Calvin disappears and Kendall is later found unconscious and with no recollection of what happened, a common occurrence with those who have confronted the Yellow "M" head-on.
Macomber, Calvin, Vernay and Septimus were all involved in the court proceedings and Thornley suddenly died of apoplexy when he lost the case.
Later that day Blake receives a letter from Septimus begging him to go to Limehouse Dock where someone will give him vital information regarding the case.
After Blake has left, Mortimer is visited by Stone from the Daily Mail who has found a copy of The Mega Wave which was sent to the paper for review when first published.
The police give chase but, despite being shot at, falling into the river, crashing his car and catching fire, the Yellow "M" still manages to escape, apparently unharmed.
Arriving at the scene by taxi, Mortimer stays on the Yellow "M"'s trail, following him into the sewers where he finds the mystery man's secret lair.
When the Yellow "M"'s cumbersome headgear is removed, Mortimer is astonished to recognise his old enemy Olrik, but the man who was once a ruthless adventurer, gang leader and master criminal now appears to be nothing more than a pathetic slave in a state of hypnosis under Septimus' control.
Now prisoner, Mortimer listens as Septimus explains how he was the writer of The Mega Wave (using the pseudonym Wade) and how the theories it put forward were the subject of ridicule by Macomber and Vernay in the press, who saw it as both nonsense and harmful to the public.
In Africa he met a white madman found wandering the desert and decided to use him as a guinea-pig in order to try out his theories on the Mega Wave and get his revenge.
In time he invented a machine called the Telecephaloscope which enables him to gain control over a subject's Mega Wave and thus manipulate them from a distance.
Septimus is still unaware that Pig, the name he gave to his subject, is Olrik, the infamous adventurer, driven almost completely amnesiac due to the events surrounding the mystery of the Great Pyramid.
There he puts on a show of Macomber, Vernay and Calvin, their minds under his control, falling on their knees and "begging" forgiveness for their attacks on him and his theories.