Indians needed to be taught obedience. So believed Sir Michael O’Dwyer, Lieutenant Governor of Punjab. What happened next shocked the world.

On April 13 1919, an unauthorised political gathering was planned for Jallianwala Bagh in Amritsar. Brigadier General Dyer marched his soldiers into the walled garden, leaving his armoured cars to block the only significant exit. Thousands of men, women and children were trapped inside. Dyer ordered his men to open fire, turning their guns on the thickest parts of the crowd. For ten minutes, they continued to shoot, using 1650 bullets. Not a single shot was fired in retaliation.

According to legend, a young, low-caste orphan named Udham Singh, was injured in the massacre. Trapped in the Bagh till morning, he was forced to hear the cries of the dying. At dawn, Udham Singh is said to have picked up a handful of blood-soaked earth, smeared it across his forehead and vowed vengeance. No matter how long it took, no matter where it took him, he would find the men responsible and kill them with as little mercy as they had shown his people.

The truth is more complex but no less dramatic.

Udham Singh would travel around the world learning the assassin’s craft. It would take him almost two decades but in 1940, he would get his revenge. His act was so audacious, it would strike at the very heart of the Empire. It would also trigger a state sanctioned cover up. Udham Singh shines a devastating light on one of the Raj's most horrific events, but reads like a taut thriller, and reveals some astonishing new insights into the truth behind the legend.