The Origin of Captain America / Wreckers Among Us / Enter Red Skull
Agent 13 works undercover during Captain America's investigation into a wave of wartime sabotage. She poses as a reporter to slip into the dressing room of Sando and Omar, a popular stage act whose supposed ability to predict the future, showing coming events in a crystal ball, has drawn the hero's suspicion after they foretell the destruction of a US tank envoy.
Her cover collapses when the act turns out to be a front for Nazi saboteurs, and she is caught in the danger, needing Captain America and Bucky's protection as they battle the spies. Once the enemy agents are rounded up, she reveals that she was never a reporter at all, introducing herself as a government agent known only as Agent 13.