Fred Leuchter builds machines for a living: electric chairs, gas chambers, lethal injection systems, sold to American prisons that need their execution equipment to work without malfunction. That expertise makes him the obvious expert witness when Holocaust denier Ernst Zündel goes on trial in Toronto and needs someone to inspect the ruins of Auschwitz. Leuchter travels there with a hammer and a video camera, chips samples from the walls, and returns to write a report declaring the gas chambers could not have functioned as described. Errol Morris interviews Leuchter directly, letting him narrate his own unraveling in his own words, while historians and chemists dismantle the report's methodology piece by piece. The film also lingers on Leuchter's stranger fixations, the pride he takes in his craft, his marriage to a woman he meets through the case, and the friendlessness that follows once the scientific community and his clients abandon him. Morris treats him less as a monster than as a case study in how a competent, ordinary man convinces himself of something monstrous.