In a distant corner of the Multiverse lies Mirrodin, a plane made entirely of metal. Blades of golden grass in the Razor Fields chime in the wind. Huge misshapen boulders are magnetically suspended above the peaks of rusted iron mountains. Elsewhere, a wide sea of silvery liquid reflects the sky like a rippled mirror. On the horizon, five suns rise and set in strange orbits: one for each of the five colors of mana.

The artificial, metal plane of Mirrodin was created by the planeswalker Karn. It was originally called Argentum. Karn also created an artifact warden to watch over the plane while he explored the multiverse. Karn brought the planeswalker Jeska to visit his "mathematically perfect" plane, but while there the two of them unwittingly introduced a mysterious contagion to the plane—a contagion that in time could transmute metal to flesh and flesh to metal. The contagion infected the warden, who in time began to call himself Memnarch and obsessed over the absence of his creator. The contagion also infected the plane, destabilizing its interior core of pure mana and giving rise to strange metal growths called mycosynth.

The deluded Memnarch devised complex artifacts that would ensnare beings from nearby planes and transport them to Mirrodin. He transformed Mirrodin into an artificial ecosystem that could support organic life. His hope was to find a being with the planeswalker spark and seize it for himself somehow. Only then could he leave the plane and seek out Karn. Memnarch's schizophrenic mindset functioned as a kind of arcane proxy, keeping other planeswalkers—even Karn—from entering Mirrodin. In other words, Memnarch's delusions partially supplanted reality.

The elf Glissa Sunseeker had the planeswalker spark and was therefore Memnarch's target. Through her own guile and power and well as the help of her allies—the goblin Slobad and the old iron golem Bosh —Glissa defeated and destroyed Memnarch.

With the twisted warden gone, Karn could reenter his plane and restore order. The denizens of Mirrodin were returned to their home planes, and Karn transformed the remains of Memnarch into the metal sphere they had once been: the Mirari. Only Glissa, Slobad, and the undead head of the human Geth remain on Mirrodin as caretakers of its strange simulacrum of an ecosystem.