Having been interested in aviation since he was a boy, Plagis volunteered for the Southern Rhodesian Air Force (SRAF) soon after the outbreak of war in September 1939. He was turned down because he was the son of foreign nationals and therefore not a citizen, despite having lived in Rhodesia all his life. After Italy invaded Greece in late October 1940, bringing the Greeks into the war on the Allied side, Plagis applied again—this time to join the Royal Air Force, which had absorbed the SRAF in April 1940—and was accepted. Training first in Southern Rhodesia, then England, Plagis passed out with the rank of flight sergeant in June 1941 with above-average ratings in all of his flying assessments.