On 21. March 1945, a mysterious accident transforms one of the Second World Wars most complex air strikes into a disaster when British bombers attack Shell House, the Gestapos Head Quarters in Copenhagen. The intension was to destroy the Gestapos archives which contained extensive information, enough for the Nazis to crush the Danish Resistance Movement. But by mistake, some of the British aircraft bomb a school in Frederiksberg where 86 children are killed; in total the attack costs the lives of nearly 200 people, the greatest loss of life on a day during the occupation. With the help from the Leading Navigator on the attack, Edward Sismore DSO, DFC, AFC, experts, eye witnesses and survivors, this documentary tries to recreate the event and determine what went wrong.

21. marts 1945 angriber engelske fly Gestapos hovedkvarter i København for at redde modstandsbevægelsen. Men noget går galt, da en pigeskole bombes på Frederiksberg, og resultatet er besættelsens blodigste dag. ANGREBET PÅ SHELLHUSET genskaber angrebet, og afslører for første gang, hvad der gik galt.