Plagge graduated from the Technical University of Darmstadt in 1924 with a degree in engineering. On being drafted into the Heer at the beginning of World War II, he was put in command of an engineering unit, Heereskraftfahrpark 562 ("Army Vehicle Park 562"; HKP 562), which maintained and repaired military vehicles.
In July 1941, during the German invasion of the Soviet Union, HKP 562 was deployed to Vilnius (Vilna), Lithuania. Plagge soon witnessed the genocide being carried out against the Jews of the area. Plagge would later testify that "I saw unbelievable things that I could not support...it was then that I began to work against the Nazis".
He did what he could to help some of Vilnius beleaguered Jews by giving work certificates to Jewish men, certifying them as essential and skilled workers regardless of their actual backgrounds. This kind of work permit protected the worker, his wife and two of his children from the SS sweeps carried out in the Vilna Ghetto in which Jews without work papers were captured and killed at the nearby Paneriai (Ponary) execution grounds. Plagge issued 250 of these life-saving permits to men, many of them without mechanical skills, thus protecting over 1,000 Jewish men, women and children from execution from 1941 to mid-1944.
Plagge supported his workers' survival by procuring extra food rations and supplying hot meals to the workers in his workshops (an unusual measure) to supplement their starvation rations. He also allowed his Jewish male workers to barter for food with his men as well as local gentiles within the workshops so that they could smuggle food back to their families in the ghetto (an illegal activity). Plagge also aided the survival of the Jews under his jurisdiction by providing warm clothing, medical supplies and firewood all scarce commodities. On several occasions he and his subordinate officers helped secure the freedom of some of his workers or their family members when they were arrested during SS sweeps of the ghetto.
In September 1943 it became clear to Plagge that the Vilna Ghetto was soon to be liquidated. All the remaining Jews in the ghetto were to be taken by the SS, regardless of any working papers they had. In this crucial period Plagge made extraordinary bureaucratic efforts to form a free-standing HKP562 Slave Labor Camp on Subocz Street on the outskirts of Vilnius. Evidence shows that he not only tried to protect his productive male workers, but also made vigorous efforts to protect the women and children in his camp, actively overcoming considerable resistance from local SS officers. On September 16, 1943, Plagge transported over 1,000 of his Jewish workers and their families from the Vilna Ghetto to the newly built HKP camp on Subocz Street, where they remained in relative safety. Less than a week later, on September 23, 1943, the SS liquidated the Vilna Ghetto. The rest of Vilna's Jews were either executed immediately at the nearby execution grounds in the Paneriai (Ponary) Forest, or sent to death camps in Nazi occupied Europe.
The conditions in the HKP camp were relatively benign, especially when compared with those in other slave labor camps across Nazi occupied Europe, with tolerable work conditions, and food at subsistence levels. Plagge ordered respectful treatment of the slave laborers and their families, instructing his officers that "the civilians are to be treated with respect". These instructions, transmitted through the example set by his subordinate officers, resulted in very little abuse by the men of his unit and the Lithuanian police who guarded the camp. In spite of the general benevolence of Plagge and his men, the SS controlled the ultimate fate of the HKP laborers.
On July 1, 1944, Major Plagge entered the camp and made an informal speech to the Jewish prisoners who gathered around him. In the presence of an SS officer he told the Jews present at his speech that he and his men were being relocated to the west, and that in spite of his requests, he did not have permission to take his skilled Jewish workers with his unit. However, he said that they should not worry, for they too would be relocated on Monday July 3, and that during this relocation they would be escorted by the SS, which as they knew was an organization devoted to the protection of refugees.
With this covert warning from Plagge, over half the camps prisoners went into hiding before the SS death squads arrived on July 3, 1944. The 500 prisoners who did appear at roll call were taken to the forest of Paneriai (Ponary) and shot. Over the next three days the SS searched the camp and its surroundings. They found half of the missing prisoners, took them to the camp courtyard and shot them. However, when the Red Army captured Vilnius a few days later, some 250 of the camps Jews emerged from hiding.