"Silver Age" was a twelve part storyline that ran through a series of one shot comic books published by DC Comics in 2000.
Each of the 12 issues was a one-shot (featuring issue #1 on the cover), but they formed a larger story arc in which the Justice League of America fights the Injustice League formed by villain Agamemno.
The art, dialogue, narrative style and even the format of the comics (larger page-counts, half-page advertisements, etc.) were deliberately anachronistic for the time of publication; thus, the issues served as a tribute, and in some cases a gentle satire, to the books and creators of DC Comics during the Silver Age of Comic Books.
Issues
All the issue titles were prefixed with "Silver Age", to distinguish them from the monthly series DC produced, even though only the Flash and Green Lantern were in print at the time (though starring later incarnations of the heroes). The current Justice League book was then titled JLA and Teen Titans was being published as simply 'The Titans'.
Silver Age
by Mark Waid and Terry Dodson
The spacefaring villain Agamemno travels to Earth where he finds Lex Luthor and explains his scheme for universal domination. The villain hopes to achieve his goal by combining three artifacts: the Absorbascon from Thanagar, a piece of jewel kryptonite and the Central Power Battery of the Green Lantern Corps on Oa.
It is explained that Agamemno needs the power of the JLA to collect these items (partially because two of them have these objects). With Luthor's advice, Agamemno assembles a team to oppose the Justice League (the Injustice League) and, using his powers, swaps the minds of the heroes and villains:
Lex Luthor swaps bodies with Superman, Chronos swaps bodies with the Atom, Black Manta swaps bodies with Aquaman, Catwoman swaps bodies with the Black Canary, Doctor Light swaps bodies with the Martian Manhunter, Mr. Element swaps bodies with the Flash, the Penguin swaps bodies with Batman, Felix Faust swaps bodies with the Green Arrow, and Sinestro swaps bodies with Green Lantern.
The villains are promised Earth in exchange for aiding Agamemmno and, after imprisoning the Justice League, they split up and head off to collect the items.
The League manages to escape and, expecting the villains to return in their bodies, initiates a plan to discredit themselves. The deceived Snapper Carr contacts other (Silver Age) heroes to track down the "villains".
Silver Age: Justice League of America
by Mark Millar and Scot Kolins
The Injustice League, in the bodies of the JLA, split up and head into space to collect the items Aggammno requires. Catwoman (Black Canary), Sinestro (GL), Dr Light (Martian Manhunter) and Mr. Element (Flash) travel to Oa where, taking the Lanterns by surprise, they are able to destroy their power rings and steal the central battery.
Black Manta (Aquaman), Faust (the Green Arrow), Chronos (the Atom) and Luthor (Superman) travel to an alien planet where they discover a crashed ship that once belonged to Brainiac. Inside are a number of shrunken cites (similar to Kandor) one of which (Rikkon Dor) contains the jewel kryptonite. Chronos shrinks into the city and gets the kryptonite. He has a moment of conscience and wants to save the shrunken inhabitants (the Daxamites) but the rest of the villains smash the shrunken cities.
Silver Age: Challengers of the Unknown
by Karl Kesel and Drew Johnson
The Challengers get the call from Snapper Carr and hearing of a break-in at Ivy University by Chronos they head out. In the university the Atom (Chronos) is accessing a colleagues "time pool" to attempt to change the recent past. The Challengers attack the Atom, who uses his shrinking ray on them, but fails to stop their attack. In the middle of the fight Prof manages to deduce that Chronos is in fact Ray Palmer and the fight ends, but strange creatures from the far future emerge from the time pool. These creatures incapacitate the Atom but the miniature Challengers triumph.
Fearing the Challengers will be destroyed before the Atom can enlarge them they are placed in the time pool where time will not pass for them. There they see their future selves (as seen in DC continuity). They also see a glimpse of strange heroes who will defeat the Injustice League. Once restored the Challengers (and the Atom) head off to find these heroes.