It was about a future Earth populated by giant animal-shaped reproducing robots, which had 'evolved' to be just like sentient animals (apart from being made of metal rather than flesh and blood). The most revered robot was the elephantine 'wheeldebeast' Amok: "Ancient, battle-scarred, ferocious, haughty, red eyes glaring from beneath a royal brow, this was Amok the God-beast. The fire from his trunk could fill a lake, one hoof destroy a fort. Twenty robots could play on his back, a hundred find shelter in his shadow. The sound of his trumpeting was delight to all creatures, and this very lowing was enough to put his cows in calf" (2000 AD prog 483).
The series is named for the future geological era in which the events take place, and following the naming convention of previous eras such as the Paleozoic (old life) and Mesozoic (middle life) eras, whose suffix derives from the Greek zoion, meaning 'animal'.
Metalzoic's first appearance in 2000 AD was in prog 483 in August 1986. Readers were introduced to Armageddon, a robot primate who had operated on his brain and "burnt out any sense of decency and pity" to replace them with "brutality and a lust for power," in order to make his tribe, the Mekaka, "the greatest tribe on Earth." The Shamek (i.e. shaman) of the Mekaka predicted the imminent return of the God-beast and the other wheeldebeasts, which would bring "many fat cows and fine young bulls with health," so Armageddon set off to find them. Meanwhile, two humans entered the story: Jool, who used to travel the solar system in a spaceship, and her partner Ngila, a farmer. Ngila was killed by a huge metal
monster, which in turn was killed by Armageddon, who then saved Jool from being eaten by a huge plant.
He took her back to the Mekaka, and the Shamek told her how "for millions of years, Inti -- the great robot god within the earth -- slept while the humans ruled this planet... Until his children, the robots, were born. Then he awoke and, in the night of blood, destroyed the tender-brains!"
Jool contradicted him, saying that in fact the Earth's magnetic core had reversed, so that "the magnetosphere surrounding the planet failed to deflect dangerous cosmic rays." Most organic life became extinct and robots were left to rule the Earth. The Shamek wanted Jool killed for insulting Inti. Then Amok and the wheeldebeasts turned up and killed Armageddon's mate, Koola.
Jool showed Armageddon and the other Mekaka a shortcut to the pits of Zinja, where Amok and the wheeldebeasts were heading, and where she lived after she crashlanded on Earth. The Mekaka arrived and got into an argument with the humans who lived there. Then Amok arrived too and started burying the rest of the wheeldebeasts. Jool realised that the pits of Zinja were the wheeldebeasts' graveyard. Amok wanted to bury his herd rather than have a young upstart supplant the ageing tyrant as leader.
Amok ate Armageddon. Armageddon drew on the power of the Earth (in a manner similar to that of another of Pat Mills' characters) and used it to reconstitute himself. He downloaded the 'master program' containing the origins of the world of robots from Amok's brain. Amok died.
Armageddon then discovered that Amok had in fact buried his herd to protect them from a meteorite storm. He and the other characters sheltered from the storm in the pits of Zinja. When they emerged, Armageddon was the new God-beast, ruler of the wheeldebeasts and, ultimately, of the world.
The story ended with Jool quoting Pythagoras: "All things are changing, nothing dies... The spirit wanders, comes now here, now there, and occupies whatever frame it pleases... From beasts it passes into human bodies, and from our bodies into beasts... But never perishes."
The last prog Metalzoic appeared in was 492, in October 1986.