Action Comics #6

Synopsis

Superman from five years in the future and the adult Legion of Super-Heroes arrive in the early days of Superman's career to recover the kryptonite stolen from the spaceship that brought Superman to Earth. When they discover that the kryptonite is hidden in a tesseract implanted in Superman's brain and is to be auctioned off, the Legionnaires use their time bubble to go back into the immediate past, disguise themselves, and recover the kryptonite to return it to the spaceship.