An artificially created duplicate of the planet Krypton, (along with its own duplicate of Krypton's red sun), populated by android duplicates of the Kryptonian civilization as it was just prior to Krypton's destruction. It was made to serve as a decoy for a malevolent fleet of alien ships that sought to invade Krypton.
The origins of Krypton-II came about as a result of several Kryptonian scientists learning of an alien invasion fleet approaching Krypton. Having no great defensive means of battling the malevolent alien armada, the great Kryptonian scientist, Jor-El, the father of Superman, devised the idea for Krypton-II. Creating a device known as an Atom-Scan Ray, Jor-El beamed a projection of a model of the planet Krypton into space, where flocks of meteors were drawn toward the projection and smashed together to form a planetoid that exactly resembled Krypton. An artificial red sun was then also created and placed near Krypton-II.
Having now created the planet, an android of virtually every person in Kryptonian civilization was created—having been programmed to think and act like their living prototypes. Among these android duplicates were duplicates of Jor-El, his wife Lara, their pet dog Krypto, the Kryptonian actress (and lost sweetheart of Superman) Lyla Lerrol, the Kryptonian film director Zik-Tul, and Kir-Du, the Kryptonian in charge of Krypton's Weather Control Tower. Since the androids did not need to breathe, they were transported to Krypton-II by means of a space barge. The animals, plants and natural wonders of the planet were created in exact detail as well.
Following the creation of Krypton-II, a large smokescreen was sprayed around the actual Krypton, to hide it from the approaching alien armada and lure them to the decoy world. In the meantime, the androids on Krypton-II, as part of their programming, set about constructing a series of complicated booby-traps all over the artifical planet—all of which were set to go off should any aliens arrive on the planet. Examples of these devices include the Kryptonian Fire-Fighting Robots programmed to attack various Kryptonian buildings and structures, an incendiary bomb in Jor-El's laboratory set to explode should anyone consider taking his "secret files", and a Weather Control Tower that was programmed to bring about a devastating shower of giant colored hailstones.
However, as the alien fleet approached Krypton-II, most of the ships were destroyed in space due to a "whirlpool of energy." In actuality, only a handful of alien craft ever landed on Krypton-II, and were quickly destroyed after landing in an immense Vibrating Valley that shook the ships to pieces, killing the occupants and rendering the craft unusable.
Krypton-II was then left to drift on though space along with its artificial red sun--having fulfilled its purpose in saving Krypton. Its creators were not conscious of the possibility of it colliding with a populated world and many years after the destruction of Krypton, Krypton-II and its red sun eventually entered the solar system of Earth, threatening Superman's adopted homeworld.
As he flies out into space, Superman takes note of the runaway planet's similarity to his destroyed homeworld. However, he is soon forced to land on its surface due to the presence of its red sun, draining him of his powers. Realizing its resemblance to Krypton, an overwhelmed Superman comes to believe his homeworld has somehow returned to existence. As he tries to investigate further, Superman inadvertently sets off many of Krypton-II's booby traps, bringing great destruction across the planet. Believing himself to be the cause of Krypton's second destruction, Superman, in a half-maddened and understandably emotional state, vows to perish on this world. However, shortly before the death of the android Lyla Lerrol, Superman is given a microfilm projector.
The microfilm projector contains a recording of his father, Jor-El and details the creation of Krypton-II. A later entry by the android Jor-El, tells of the arrival of the alien ships. Relieved that he has not brought about the mass-murder of his people and coming to his senses, Superman then faces with the daunting task of how to prevent Krypton-II from destroying Earth. Fortunately, he learns that the planet-wide booby trapsa he has sprung have knocked Krypton-II out of orbit, sending it hurtling into its artificial red sun. Using one of the undamaged alien craft, Superman manages to escape the destruction of Krypton-II. Returning to Earth, his powers return shortly afterward, however, the experience of meeting the android Lyla Lerrol, causes Superman to miss his Kryptonian sweetheart all the more (S No. 189, Aug 1966: "Krypton Lives Again!"; "Krypton's Second Doom!").