In Adventure 334 (July 1965) Edmond Hamilton crafts a decent mystery story, though anyone familiar with story structure would figure it out, even before the mystery begins.
The most enduring element of this tale is actually a location in it, the Prison Planetoid, which appears for the first time. Later, this location would acquire the name Takron-Galtos, and be a frequent location in Legion stories.
The Legion are tracking an escaped felon, Dr Eldor, and follow him to Antares, the planet of the Proteans. He was the one who gave them their powers, and they now use those abilities to shield him, and fight the Legion on his behalf.
A mysterious helmeted hero comes to the aid of the Legion, and much of the story is devoted to figuring out who it is. Mon-El and Ultra Boy are the two most obvious suspects, based on the range of powers the hero has, and the fact that Superboy is involved in the story, so not a viable suspect.
Sadly, the solution is a bit of a let-down, a bit of deja-vu. It’s Supergirl, with red kryptonite-induced memory loss. Shades of Satan Girl. In the opening pages of the tale she headed back to Stanhope College. The simple fact that she was in those pages, and had no function in the tale, alerts any experienced reader that she must return and serve some purpose.