Wonder Woman and Batman arrive at Superman's Fortress of Solitude aboard Wonder Woman's new invisible Javelin, each of them bearing presents for Superman's birthday. They find him in a coma and covered by an alien plant teleported in as a gift. As Batman investigates, Wonder Woman is knocked out by Mongul who confronts the Darknight Detective. Wonder Woman gets the drop on him but Batman intervenes, getting Mongul to brag that the plant is a Black Mercy, which feeds the victim a beguiling mental illusion while leaving him in a coma. Wonder Woman fights against the superstrong Mongul while Batman tries to remove the plant.
On Krypton, Kal-El wakes up with his wife Loana at their farm, as the landscape is shaken by a quake. Their son Van-El and his pet dog Krypto come down and Kal-El takes his son to see his grandfather Jor-El as quakes continue to rock the planet. Jor-El reveals he was wrong in his prediction of years past that Krypton would be torn apart. He begins to realize that something is wrong as Batman tries to get through to him. Kal says goodbye to his son and forces himself back to the real world as "Krypton" is destroyed once more.
The plant detaches itself and wraps around Batman, granting him his greatest wish – that his parents had survived their shooting when Bruce's father defeated the gunmen. An enraged Superman attacks Mongul, who has Wonder Woman on the ropes. The two fight while Wonder Woman manages to free Batman. She manages to get through to him and he also rejects the dream and returns to reality, loosening the plant. Wonder Woman manages to fend it off and throw it onto Mongul as he prepares to kill Superman. They wish Superman a belated Happy Birthday as they wonder what Mongul is dreaming of.
Kandor is the bottle city in the Superman comics - a part of Krypton that was shrunk and placed in a bottle by Brainiac (first seen in Action Comics #242 - July 1958). Superman later got hold of it and it became part of the huge pre-Crisis Kryptonian background. Post-Crisis, it first appeared in Superman #107 (December 1995) and was revealed that it was a ghetto on Krypton where aliens were banished by the Pure Krypton Cultural Program. It ended up shrunk and dimensionally locked into Limbo and then came into the possession of an alien sorcerer, Tolos, and later Superman got hold of it.
In the comics pre-Crisis (Adventure Comics #210 - 1955), Krypto is Superman's pet who was sent by Jor-El in a smaller rocket as a test drive before sending his own son Kal-El to Earth, but was lost when a meteor hit the ship. The dog then appeared on Earth to a teenage Clark Kent who was at the time Superboy. Krypto later traveled to the future and joined the Legion of Super-Pets. Post-Crisis (in Adventures of Superman #501 - June 1993), Krypto was the product of a alternate Krypton from the Phantom Zone, who travelled to our reality after Superman and Lois came back from their journey to the Phantom Zone Krypton.
Zod is the Kryptonian general who at one time wanted to rule Krypton with an iron hand but was stopped by Jor-El. In the Superman Movie he was sentenced to the Phantom Zone and put there by Jor-El, He later appear in the second Superman movie with the other two Kryptonian criminals and tried to rule Earth but was stopped by Superman. In the comics pre-Crisis (starting with Adventure Comics #283 - April 1961), he was imprisoned by Jor-El in the Phantom Zone, and plotted on many occasions against his enemy's son. Post-Crisis (starting in Adventures of Superman #444 - September 1988), he was imprisoned as above and later escaped to an alternate universe and wreaked all kinds of havoc there. Superman executed him using kryptonite after a Supergirl created there went to this universe and asked Superman for help in defeating him. Years later a new Zod in red armor who was the ruler of a place called Pokolistan made his debut and challenged Superman in all-out war that ended with his death.
Fortress of Solitude
This is the first time we see the Fortress of Solitude in the Justice League series. The Fortress of Solitude is another longstanding feature of the Superman mythos, although it may or may not have been inspired by a similar "Fortress of Solitude" (circa 1938) possessed by the pulp hero Doc Savage. In any incarnation of Superman (dating back to Action Comics #241 - June 1958), the Fortress has been Superman's refuge from the world, a hidden base located in the Arctic (except for a few times when the writers screwed it up and put it in Antarctica!). The equivalent of Batman's Batcave, Superman keeps many Kryptonian artifacts, including the Bottle City of Kandor, all recovered from the remains of Krypton, as well as trophies from his case, a vast scientific laboratory, an alien zoo, etc.
Van is probably taken from Van-Zee (first appearance Superman #158, first series - January 1963). Pre-Crisis, he was Kal-El's lookalike cousin who lived in the bottle city of Kandor. He worked as Nightwing, the "Batman" of Kandor and was a member of the Superman Emergency Squad.
This is J.M. DeMatteis' first work on a WB animated series, but he had a good run of episodes on another Superman series: The Adventures of Superboy (1988-1992). Matteis is best known for his comic book work - he worked at Marvel on The Defenders and Captain America, wrote a graphic novel (Moonshadow) for Marvel's Epic line, then moved to DC and took over Justice League scripting duties. Post-Crisis, he and Keith Giffen created the new, light-hearted Justice League, and has since bounced back and forth between the two companies. His most recent work is the revival of "his" Justice League in Formerly Known as Justice League.