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This achievement is missable and is earned at the start of Mission 16, Stopping Marchenko / Mission 17, Protecting the Future.

Provided you completed Mission 12, The Heist, during your playthrough, you will have in your possession the Orchid Antidote. After speaking with Miller at the end of Mission 15, you will be tasked with stopping Marchenko or protecting the delegates. Before you leave the kitchen to tackle one of these objectives, speak with Miller again and give him the Antidote to save is life and unlock the achievement.