Alternative versions of Superman establish a new continuity for DC, affecting not only Superman, but all of the DC Comics characters. The versions of Superman from this time period are traditionally divided into three main periods. The first version of Superman began being published by DC Comics in 1938 appearing in "Action Comics" #1. To explain discrepancies in the aging of Superman across several decades, his earliest stories were retroactively portrayed as having taken place on an alternative world called Earth-Two. These stories take place from 1938 until the late 1950s, although the exact dividing line is unclear and some stories happened identically to
Superman/Batman Batman discover each other's secret identities and agree to work together, even though they disagree with each other's methods of operating. The issue is a reimagining of "The Mightiest Team In the World", the tale from "Superman" #76 (May–June 1952) in which the two heroes discover each other's secret identities and team up for the first time. In "The Unexamined Life... " ("Annual" #2 (2008)), Superman loses his powers and takes on the identity of Supernova. The issue is a reimagining of "The Has-Been Superman" from "World's Finest Comics" #178 (September 1968) and "Superman's Perfect Crime" from "World's Finest Comics"