Inside their Secret Sanctuary, the Justice League of America answers their mail. A letter from millionaire philanthropist, Bruce Wayne, peaks Superman's curiosity. Among his fellow members of the Justice League of America, only Superman knows that Wayne is, secretly, the Batman. Intriguing letters from physicist Ray Palmer, and the mysterious C. King spur the team into action. The Flash, Green Arrow, and Snapper Carr journey to Gotham City, to follow up with Wayne. Upon confronting him, however, Wayne has no recollection of ever sending a letter to the Justice League of America. The investigation into the crimes of a, supposedly, invisible thief is the case Wayne is currently working on, as the Batman. Wayne, however, never intended to involve the Justice League of America.
The Flash, Green Arrow, and Snapper Carr follow up the investigation, at the scene of the crimes. Unbeknownst to them, Wayne will be joining their investigation, as the Batman. Wonder Woman and J'onn J'onzz, the Manhunter From Mars, meet with Palmer, to discuss the curious case of a self-firing cannon. Palmer also claims to have never sent a letter to the Justice League of America. Wonder Woman and the Martian Manhunter make their way to the cannon. Unbeknownst to them, Palmer will be joining their investigation, as the Atom. Superman and the Green Lantern make contact with C. King, regarding his letter concerning a disappearing island. King has no knowledge of any letter sent to the Justice League of America. Superman and the Green Lantern head out to find the disappearing island.
Unbeknownst to them, King will be joining their investigation, as Aquaman. The Flash, Green Arrow, and Snapper Carr meet with Jason Markham, the millionaire art collector. Already robbed on multiple occasions, Markham fears that his latest acquisition will be the next item stolen. That night, the Flash circles Markham's estate, at super-human speed. The Batman keeps a watchful eye from the roof. A masked figure prowls through the Markham estate. The Green Arrow, and Snapper Carr, spring out from inside the vault, only to be gassed into unconsciousness. The Flash and the Batman subdue the masked figure. Before they can unmask him, however, they are seized, along with the Green Arrow, and Snapper Carr, by tentacles, reaching out to them from a rift in the time-space continuum.
In Ivy Town, the self-firing cannon expels a shell. J'onn J'onzz, the Manhunter From Mars, blocks the cannonball with his invulnerable chest. The Atom is just about to solve the mystery behind the cannon, when more tentacles emerge from a gap in the space-time continuum, grasping the three heroes in their inescapable grip. Almost simultaneously, Superman, the Green Lantern, and Aquaman solve the secret of the disappearing island. Before they can compare notes, they too are grabbed by the extra-dimensional tentacles. The Justice League of America is drawn through the rift in the space-time continuum, and brought before the alien entity known only as "I". Inexplicably, every victory achieved by the Justice League of America has pushed "I" to the brink of oblivion.
Should the Justice League of America succeed on just one more case, "I' will become totally inert.The Justice League of America attack "I' en masse, to no avail. Before bringing the Justice League of America into it's home dimension, "I", inexplicably, robbed them of their ability to succeed together, as a team. "I" returns the Justice League of America to Earth. No longer able to work together, the Justice League of America becomes despondent, and considers disbanding. Snapper Carr, however, determines a way to resolve the crisis. Snapper Carr recommends reactivating the android Amazo, known as the "One Man Justice League". Using the combined powers of the Justice League of America, in concert against the team, Amazo's triumph is the final straw that ends the threat of "I" forever.
With the Justice League of America restored to normal, the team takes down Amazo, then deactivates the android villain. Amazo is returned to the Justice League of America's trophy room. With the threat of "I" ended, the team compares notes on the cases they were working on. The Batman reveals that the "invisible thief" stealing from Markham, was Markham himself. Walking in his sleep, Markham was moving his own paintings, to a new location, to ensure their safety from theft. The Atom explains the science behind the self-firing cannon, and how it's discharges were timed, to cover the explosive charges being used by a ring of bank robbers. J'onn J'onzz, the Manhunter From Mars brought the criminals to justice.
Finally, Superman, the Green Lantern, and Aquaman reveal that the "disappearing island" was really an inflatable cover, for a submarine, used for smuggling. Snapper Carr looks for the three letters that initiated the case, to add to the trophy room. The utter destruction of "I', however, has unmade all of the entity's creations. The letters no longer exist. Snapper Carr must be content with his memories of this particular case.