Julia Koenig first appears in a multi-issue storyline in the title Invaders, posing as a servicewoman dating an American soldier in London during World War II. Koenig is revealed to be a Nazi spy, code named Madame Rätsel (Madame Mystery), who is sent to obtain information from a soldier who was also a cartoonist, and is suspected of knowing the secret of the Super-Soldier Serum, which originally empowered hero Captain America. The soldier is captured and interrogated by Koenig, who attempts to replicate the experiment in a laboratory. The materials used are unstable, and when a superior officer intervenes, trying to stop Koenig using the formula on herself, she whips him, accidentally throwing him into the machinery, causing an explosion that transforms Koenig into a female version of fellow Nazi agent Master Man (although courtesy of a higher level of exposure to the Serum, Master Man is physically superior). Koenig dubs herself Warrior Woman.