The JLA and JSA, alongside the Legion of Super-Heroes, are captured by Mordru in the 30th century and forced to do his task: reclaim the three items lost after the JLA satellite was destroyed in the late 20th century.
The Justice Society of America prepares to take the Psycho Pirate back to Earth-2. The Green Arrow delays their departure for an hour, nearly coming to blows with Wildcat. Power Girl, however, is delighted to stay, having taken a shine to Superman. While taking a tour of the Justice League of America's trophy room, Superman and Power Girl are attacked by a massive, disembodied hand. The Justice League of America, and the Justice Society of America, rush to the Kryptonians' aid. The hand blindly gropes about, accidentally grasping five members each of the two teams.
The Justice League of America, and the Justice Society of America, find themselves drawn 1,000 years into the future, by the sorcerer, Mordru. Having no use for either group of heroes, Mordru imprisons them, as he has already done to the Legion of Super-Heroes. Doctor Fate combines his eldritch power to that of the Golden and Silver Age Green Lanterns, to break free from Mordru's cage. Doctor Fate challenges Mordru. As Doctor Fate falls, the Justice League of America, and the Justice Society of America, rise to the attack. Mordru shrugs them all off, and imprisons them once again.
Doctor Fate, though, has cast a spell of suggestion upon Mordru, making the evil sorcerer less inclined to destroy the heroes. Mordru seeks the Red Jar of Calythos, the Green Bell of Uthool, and the Silver Wheel of Nyorlath. With these three mystic objects in his possession, Mordru means to release the Demons Three--Abnegazar, Ghast, and Rath. Mordru expects to add the power of the Demons Three to his own. Mordru took the Legion of Super-Heros by surprise, and tasked four of their number with retrieving two of the mystic items.
When they failed to return, Mordru reached back through time, to steal the items from the Justice League of America's trophy room, garnering the ten heroes by mistake. Influenced by Doctor Fate's spell of suggestion, Mordru tasks the Justice League of America, and the Justice Society of America, with retrieving the three mystic items. Mordru keeps the Green Arrow, and the Black Canary, as his hostages, along with the half dozen Legionnaires he is already holding. The Green Arrow fills in the Black Canary regarding the Demons Three, and the three mystic items.
Mordru reveals that at some point, during the last century, the satellite headquarters of the Justice League of America was destroyed. The three mystic items were hurled to the far corners of the galaxy, where Mordru seeks them now. Seeking the Silver Wheel of Nyorlath, Sunboy, and Wildfire, have been taken prisoner, on Antares-II. Doctor Fate, the Golden Age Hawkman, and Superman come to their rescue. Flying so fast he is invisible to the naked eye, Superman lifts the Silver Wheel of Nyorlath into the sky. Doctor Fate makes it seem as if the Silver Wheel of Nyorlath transforms into stars.
The primitive inhabitants of Antares-II, who worshiped the Silver Wheel of Nyorlath, now worship the stars. The Batman, and the Golden and Silver Age Green Lanterns, journey to the planet, Vaxon. The upper atmosphere of Vaxon is filled with dragons. The sound of the Green Bell of Uthool seems to be keeping the dragons at bay. On the surface of Vaxon, Brainiac 5, and Princess Projectra, plead with the ruling council to relinquish the Green Bell of Uthool. The council refuses. The Green Bell of Uthool is the only thing keeping the dragons from attacking the populace.
The Batman proposes creating a scarecrow. Working from Princess Projectra's cast illusion, the Golden and Silver Age Green Lanterns carve a bas-relief of the dragon's natural predator, the Xanthor, on one of Vaxon's continental land masses. The dragons depart in terror. A grateful ruling council relinquishes the Green Bell of Uthool. The Golden Age Flash, and Power Girl, open a rift in space to retrieve the Red Jar of Calythos. The extra-dimensional pocket is filled with brooding, amphibious monsters, each squatting on an egg. Threatened by Power Girl's presence, the creatures attack.
One of the creatures has mistaken the vault holding the Red Jar of Calythos for it's egg. Power Girl brings the creature's actual egg to it. Power Girl cracks open the vault, and retrieves the Red Jar of Calythos. Mordru draws the three teams of heroes back to him, and imprisons them once again. The Giolden Age Flash escapes imprisonment. The Golden Age Flash has held back the Red Jar of Calythos. The Golden Age Flash barters the release of the prisoners for the Red Jar of Calythos. Mordru complies, Mordru initiates the ritual to release the Demons Three.
The Justice League of America, the Justice Society of America, and the Legion of Super-Heroes attack Mordru. The Silver Age Green Lantern calls a halt to the attack. Mordru still has the Black Canary, and the Green Arrow, imprisoned, in an hour glass around his neck. The heroes watch helplessly as Mordru completes the ritual, releasing the Demons Three from their imprisonment. With a gesture, Mordru causes the Justice League of America, and the Justice Society of America, to vanish. The Demons Three refuse to grant power to Mordru, opting instead to strike him down.
With the mystic items in their possession, the Demons Three can never be imprisoned again. Forgotten by all, the Black Canary, and the Green Arrow, are still trapped inside Mordru's hourglass. An hourglass that has almost entirely filled with sand.
"100 Issues Ago In...Justice League of America" reprints material from Justice League of America #47 (September, 1966). A plot summary for that story can be found at the link.