Kroagnon was a brilliant architect whose creations were well-renowned - so impressive, in fact, that he was sometimes called the Great Architect. Kroagnon was very clever and even more proud, with a great desire to be appreciated by his equals, and he also possessed a quick temper. His most wondrous architectural achievements included Golden Dream Park, the Bridge of Perpetual Motion and his crowning glory, Miracle City. But Miracle City also gained him notoriety for another reason - when it was completed, he refused to move out. Kroagnon hated to see his buildings inhabited by human beings, who promptly made a mess of them. Eventually he was removed from the building, but mysteriously, those who moved in all died. Kroagnon's involvement in the deaths could not be proven and he was not convicted. Eventually he got other work, including a luxury residential complex known as Paradise Towers. But to the surprise of the general public, Kroagnon disappeared before the completion of Paradise Towers. Armed with foreknowledge of Kroagnon's deeds, the residents of the Towers had trapped Kroagnon's brain in the basement of the building before he could finish seeding the building with killing devices in every corner. Apparently, his race is one that can survive death in a way. Kroagnon's physical form was reduced to a pair of glowing red lightbulb-shaped neon tubes surrounded by a swirling fog, hidden behind a bulkhead-like door in the basement.
But a noncorporeal existence was not enough to stop Kroagnon, who began looking for a body suitable to house his brain. He bided his time, establishing a relationship with the Chief Caretaker of the Towers. The Chief Caretaker called him "Daddy's little pet" and arranged for the Robotic Self-Activating Megapodic Mark 7Z Cleaners to surreptitiously kill off some Yellow Kangs and Caretakers to "feed" to him. Despite the caring relationship that the Chief Caretaker tried to cultivate, he was actually very afraid of his pet, and although he tried to exert his authority, he could not. One angry snarl of "Hungry!" from Kroagnon and the Chief Caretaker would instantly back down and agree to what Kroagnon wanted done. Nor were his attempts to pick off the population of the Towers such a well-kept secret - rumors and first-hand sightings abounded, and images of the door in the basement and people being killed by Cleaners were common elements in Kang wallscrawl.
Kroagnon eventually began taking matters into his own hands. He gained control of the Cleaners and directed them to start killing Red Kangs as well. But none of these bodies were right for him to live in - as it happened, the most suitable one was that of the Chief Caretaker, so Kroagnon transferred his mind to the Chief Caretaker's body using a technique known as corporelectroscopy. He apparently retained the Chief Caretaker's memories but had difficulty controlling his new body - Kroagnon's speech and movements were slurred, his tongue was apparently swollen, and his voice deepened to a gravelly growl. The skin also took on a metallic silver hue. He was dismayed to see how his Towers had become "polluted with living flesh" and began a systematic wipeout of all the humans - Kangs, Rezzies and Caretakers alike. The Cleaners roamed the halls, spraying poisonous gas at the humans and leaving the corpses where they fell for future cleanup.
The Doctor, Mel, Pex and the factions of Paradise Towers grouped together in the swimming pool area and worked together to slow down the Cleaners with crocheted tablecloths, blow them up with explosives mounted on arrowguns, and rig the Red Kangs' brainquarters as an explosive trap for Kroagnon. The Doctor's appeal to Kroagnon's vanity led him into the trap, but too soon for it to be sprung. Pex activated it manually by hurling himself and Kroagnon into the tripwire, killing them both.
