Action Comics (1938) 287-A

"Supergirl's Greatest Challenge" - - *The Legion of Super-Heroes summons Supergirl to defeat the Positive Man, a once-human changed into a massive destructive force by the explosion of his home planet. As they watch from their spaceship, the Maid of Might lures a Negative Creature, created by the same explosion, toward the Positive Man, and the resulting explosion causes both to vanish.

Upon returning to Earth, the Legionnaires learn that this explosion has also erased their powers. Supergirl decides to stay with them, and, working behind the scenes with Whizzy, the telepathic descendant of Streaky (aka Super-Cat), helps the Legion deceive criminals into thinking that they still possess their abilities.

However, the real reason that the Legionnaires are powerless is that they are actually members of an alien chameleon-like race who, in the aftermath of the positive-negative explosion, imprisoned the real Super-Heroes and took their places. Having fooled Supergirl and Whizzy, they project them into the Phantom Zone.

From here, the phantom heroes mentally command a scientist at an android factory to construct a bogus chameleon man, who infiltrates the Legion imposters and releases Supergirl and Whizzy from the Zone. Supergirl captures the aliens and rescues the real Legion. *Source: The Official Legion of Super-Heroes Index #1 - Published by ICG.

Also contains the back-up story: "Perry White's Manhunt for Superman".