Upon the United States' entry into WWII, Hitler orders General Heydrich (Kenneth Branagh) to summon select SS officers and various Reich officials to a secret meeting at a house near Wannsee. Their purpose is to accelerate and put in place the long-delayed "Final Solution" to "the Jewish question." Heydrich informs the group that since the Allied nations were refusing to take in more German Jewish refugees or were charging Germany great sums to do so, and since Germany could not feed and house all the Jews of conquered Europe, the SS had designed a plan that would remove every single Jew from Europe within a matter of years.
At first, a few find the notion absurd and more of a joke. They quickly realize the SS is deadly serious when Lt. Eichmann (Stanley Tucci) coldly reads the graphic results of a feasibility experiment on the most efficient means of rapid mass murder and corpse disposal. Others, including some of the military, are appalled that their shared hatred of the Jews could go this far. One official objects that many skilled Jews are critical to German war production but would be eliminated at the worst possible time, as there's no Germans to replace them. As the group slowly realizes all that this "removal" of Jews will entail, Heydrich spends much of the meeting subtly or overtly threatening the more reluctant attendees into cooperating.
Wilhelm Stuckart (Colin Firth), co-author of the Nuremburg laws, vocalizes his deep offense -- partly on principal, partly out of pride of authorship -- that the Solution will render all German law subservient to the caprice of the SS. Heydrich reminds the group that the Führerprinzip has already made this a fact of life and that the Nuremburg laws can and will be overridden as necessary. Stuckart desperately offers legal alternatives to deal with Germany's Jews but these are denied by Heydrich as only complicating and delaying the problem further. Eventually, Stuckart realizes he has no choice. However, while affirming to the group his personal hatred for Jews, he also warns them that no matter how much other nations may hate Jews, this Solution, if discovered, will not make the Reich the beacon to the world that the Nazis envision. It will only make Germany a pariah.
The most powerful civilian at the conference, Chancellor Kritzinger (David Threlfall), is the final holdout. He initially refuses even to listen to Heydrich on the grounds that Hitler had personally assured him that literal elimination of Jews (which had evidently been rumored) would never be an option. Heydrich gently but pointedly informs Kritzinger that his Führer lied to him and will continue to lie. Kritzinger, too, is finally threatened into compliance, his image of a glorious, law-based Reich forever shattered.
In the end, all voice their agreement with the Final Solution. Each is ordered to study and destroy the notes they received. The guestbook and all other records from the meeting are burned by Eichmann.
An epilogue reveals that one of the attendees failed to burn his copy, which were discovered when the Allies took Berlin. His notes were the only surviving record of the Wannsee conference and served as evidence for the prosecution at the Nuremburg trials.