The Legion of Super-Heroes in the 31st Century #20

Synopsis

While Brainiac 5 is distracted with other-dimensional research, someone breaks into his lab and steals kryptonite. Soon after, someone claiming to be Metallo attacks Superman, but he is rescued by his Legionnaire friends. When Metallo returns once again, the Legion exposes him as Tharok in disguise and Superman retrieves the kryptonite, returning it to Brainy, who is unaware that he too is the subject of someone else's inter-dimensional research.