The Yellow "M" by Edgar P. Jacobs

When a series of inexplicable attacks and paralysis cases strike London and are marked by a strange yellow M-shaped symbol left at each scene, a government agent and his scientist friend launch an investigation that draws them into a web of espionage, blackmail and dangerous, cutting‑edge technology. Their pursuit uncovers a ruthless mastermind who manipulates minds and systems to destabilize the country, forcing the investigators into a race to expose the conspiracy, confront betrayal close to home, and stop a plot that threatens national security.

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