The Investigation is Weiss' ruthless documentary drama of the Frankfurt Auschwitz trials, which he attended. These proceedings, held in 1963-5, are not to be confused with the Nürnberg trials held right after the war. In Frankfurt it was the German government itself that held the war-crimes trial, focussing on the crimes perpetrated at Auschwitz.

Using the actual testimony of survivors from Auschwitz, testifying as witnesses against those who exploited them and others, Weiss creates a riveting drama. The drama is based on the trial, but Weiss insists that it should not be staged as a courtroom-docudrama.