The first regular Superman/Batman team-up, in World’s Finest 217 (May/June 1973), brings in Metamorpho as well, a teaser for his own series, beginning in the following issue, which moves over from Action Comics.
It’s Java, Simon Stagg’s ape man servant, who causes the problems in this story, by Haney, Dillin and Anderson. He makes the Composite Superman suit for his own amusement, but tosses it carelessly away when he almost gets caught with it. Metamorpho is in the vat, undergoing another attempt to “heal’ him.
Somehow, this suit merges with Metamorpho, and even endows him with the powers of all the Legionnaires, somehow.
Loving his new power levels, Metamorpho shows up Batman and Robin when a nuclear capsule needs retrieving. He basks in the adulation, which is rare for this character.
Superman and Batman get so upset that the defect to the nation of Slavia. Metamorpho is sent by the state department to get them back.
There are some really fun actions scenes as the heroes battle each other. Metamopho never does clue in that the defection was a fake, designed to get information on Slavia’s weapons.
The Composite Superman powers wear off by the end of the story.