During World War II, Norway was invaded by Germany on 9 April 1940. Helge Skappel's brother died in the Battle of Stryken already on 12 April. Skappel joined the resistance movement, helping Norwegians to flee the country. He also transmitted information about German troop movements via illegal radio. He was arrested for conducting "intelligence" in June 1941, and was imprisoned at Møllergata 19 for a month. He was sentenced to six years of prison. He was transferred to Grini concentration camp in July 1941, as #353 of almost 20,000 prisoners. Viggo Widerøe was imprisoned as well, and they spent prison time in Åkebergveien from October to November 1941, before being shipped to Germany via Akershus Fortress. He was imprisoned at Hamburg-Fühlsbuttel, and remained so until being saved by the White Buses at the war's end.