On 9 April 1940, Nazi Germany invaded and occupied Norway as a part of the Second World War. Gjems-Onstad became aware of the German presence when he and his family saw German planes flying right over their home. They did not get any notification of what was happening, and only music was played on the radio. Gjems-Onstad took his bike and cycled to his school, the Oslo Cathedral School, only to find it closed and in a chaotic state. He thereafter went to the city centre and "Karl Johans gate", where he witnessed the Germans marching through. The next (or possibly the same) day, a German Junkers plane crashed near him and his friend when they were on their way from Lysaker to get a closer view of the captured Fornebu Airport. He briefly found himself in the situation of attempting to rescue the Germans trapped inside the plane, but the heat was unbearable and the two were told by the Germans to run away and get in safety just moments before the plane exploded. The proclamation by Norwegian Prime Minister Johan Nygaardsvold on 10 April (that rejected the German claim to appoint Vidkun Quisling as Norwegian Prime Minister), was according to Gjems-Onstad received with great disappointment by him and his friends. He thought it contained nothing but verbiage, with no remarks about mobilisation, defence, fighting or war. He thereafter wanted to join the resistance, but on his way met a man who told him that he would be rejected if he had not been through recruit training, and he thus returned home. After a few days, he and a friend nevertheless set out for Northern Norway. They came as far north as Nord-Trøndelag, where they however gave up and returned home.