X of the Underground was a female agent (most likely French) posing as a man, who infiltrated many Axis camps. Her efforts were complicated by reporter Bob Gray, who discovered her secret. Repeatedly she would ditch Gray only to be found again.
In her sometimes gruesome adventures, she inspired women across Europe who joined in forming a covert organization.
In her final appearance, she barely escaped a Nazi massacre and rallied her allies to take down a powerful German officer.