The Blue Dolls were based off Casey Laws and resembled her. They were small humanoids, with blue skin and simplistic faces. They had no real organs, only spongy, undifferentiated tissue and microscopic circuitry. This meant they couldn't be killed with physical force, but could be destroyed with electromagnetic weapons. They attempted to turn humans into Blue Dolls, but this was almost universally fatal, with the haphazard transformation severing arteries and veins. (PROSE: The Wheel of Ice)
When human colonists first came to Mnemosyne, Arkive used tripods to investigate them. The first human they encountered was Casey Laws, so the first Blue Doll, simply known as "First", was created in her image. Hundreds of Blue Dolls were created and sent out to collect materials to create a time machine for Arkive, as well as stop the humans from interfering. Their activities were first blamed on children, but eventually they directly encountered humans. One of the workers, James Campbell, was attacked in the mines. Another was caught when it was trying to sabotage a tractor. It was found by Florian Hart, who had a grudge against aliens, who killed it and tried to remove the evidence of its existence. The Second Doctor and Zoe Heriot found the dead Blue Doll and decided to go looking for the other Blue Dolls and its creator.
The Doctor and Zoe went down to Mnemosyne, along with Florian Hart, Sonia Paley, Jo Laws and Dr Sinbad Omar. When the Doctor found a chamber full of Blue Dolls, he wanted to learn from them, but Florian shot at one and they fought back. Dr Omar was captured and the rest could only barely escape. The Doctor tried to contact the Blue Dolls again, but Florian killed another one. In order to stop this, Arkive created the Blue Soldiers from Dr Omar's template. These Blue Soldiers were sent to kill the humans and finish the Arkive's project. The Doctor tried a third attempt to make peace with the Blue Dolls and contact their creator. Using posters with the rings of Saturn on them, the Doctor was able to convince the Blue Dolls to take him to Arkive. Eventually, the Doctor was able to make peace with Arkive. The Blue Dolls were already showing signs of age and the Doctor believed they would deactivate soon. (PROSE: The Wheel of Ice)