Eight men were waiting in the dark near 中の橋 (Nakanohashi) bridge. They had chosen their spot well — a narrow road through the 麻布 (Azabu) district of Edo, on the only route between the Prussian legation and the American compound at 全福寺 (Zenpuku-ji) temple. They knew their target. They knew his habits. They had probably been watching him for some time.

Henry Heusken was twenty-eight years old, Dutch-born, American by choice, and the most consequential interpreter in Japan. He had arrived in Nagasaki in 1856 as personal secretary to Townsend Harris, the first U.S. Consul-General to Japan, a man whose Dutch was not strong enough to conduct diplomacy alone. Heusken's Dutch — and his rapidly acquired Japanese — made him indispensable. For nearly five  years he had sat beside Harris at every critical meeting with the 老中 (rōjū), the Shogun's senior council: the first audience at Edo Castle in December 1857, the grinding months of treaty negotiation, and finally the signing of the 日米修好通商条約 (Nichi-Bei Shūkō Tsūshō Jōyaku), the Harris Treaty of Amity and Commerce, on July 29, 1858. That treaty opened four additional Japanese ports to American ships and granted Americans the right to live and trade in Japan. Without Heusken's voice in the room, it is difficult to imagine the same outcome.

The treaty was also the single most hated diplomatic document among the radical loyalists who had begun calling themselves 尊王攘夷 (sonnō jōi) — "Revere the Emperor, Expel the Barbarians." To them, the 幕府 (bakufu), the Shogunate, had committed an act of national humiliation by welcoming Western trade and foreign residence. The men who negotiated that treaty were collaborators. The man who had given it a voice was a walking symbol of everything they despised.

Heusken had already been warned. He had been stoned by a mob while riding through Edo with the Dutch Consul. He knew he was a target. He rode out anyway, on the evening of January 14, 1861, to dine with Count Friedrich Albrecht zu Eulenburg, the Prussian envoy. It was an ordinary diplomatic dinner. On the way home, the eight men struck.

Three names are recorded in Japanese sources — 伊牟田尚平 (Imuta Shōhei), 樋渡八兵衛 (Hiwatashi Hachibei), and 神田橋直助 (Kandabashi Naosuke) — reconstructed years later from private journals, never from a trial. No one was ever arrested. No one was ever charged.

The attackers' swords opened Heusken across the abdomen on both sides. He should have died in the road. Instead — through some combination of shock, will, and horsemanship — he pulled himself onto his horse and galloped the two hundred yards to the American Legation gate. Surgeons arrived from the English and Prussian missions. They worked through the night. Heusken died from blood loss at approximately 12:30 in the morning of January 16, just over a day after the attack.

The 幕府 (bakufu) paid a $10,000 indemnity to his mother in Amsterdam. That was the full extent of the formal consequences.

What followed in the diplomatic community was its own scandal. Every Western legation except one evacuated Edo for the relative safety of Yokohama. France and Britain landed troops. Secretary of State William Seward would later call Heusken's murder "yet unpunished and unatoned" in official correspondence. The sole exception was Townsend Harris — the man Heusken had served for nearly five years. Harris not only refused to leave Edo, he told his fellow ministers, according to accounts in secondary sources, that Heusken had been reckless to ride out after dark. His colleagues were furious. His dead interpreter's reputation was, apparently, a reasonable price for maintaining Harris's standing with the Shogunate.

Heusken had kept a diary throughout his years in Japan, written in French. It was discovered in an Amsterdam bookshop in 1953, nearly a century after he sailed for Nagasaki. Published in 1964 as "Japan Journal, 1855–1861", it ends mid-sentence. He never knew anyone would read it. He was twenty-eight years old, a linguist on horseback, and he had helped change the world — which is precisely what got him killed.