Mandrels are tall creatures covered in red scales and dark fur. On their furry heads are large, glowing green eyes and a fanlike mouth from which several fronds dangle. Their long arms end in clawed hands, and their legs, with bands of hair at the bottom, flare out to a grey pad underneath. Although they are not very intelligent, they make up for this with their loud roar and the vicious swipes of their claws with which they fatally wound people. It is not entirely clear how they kill people; some seem to drop dead at the mere touch of a Mandrel, perhaps indicating that the creatures are very strong. However, others are able to survive the attacks, so perhaps some of the Mandrels' victims simply had weak hearts. They are usually to be found in a violent rage, but can be calmed with a dog whistle. When thus entranced, they make a keening sound rather like whalesong. Mandrels cannot be killed by laser guns - although it does either stun them or force them to retreat - but if a Mandrel is electrocuted, it then melts down into a pile of lumpy white powder...the dangerous, addictive drug vraxoin.
Vraxoin, also known as drug XYP, or "vrax" colloquially, was originally made from a kind of fungus. It also appears as a sparkly gold powder - perhaps the refined form? When taken (internally via the oral cavity is one effective method), vraxoin induces warm complacency, then total apathy, until it wears off. In the first stage, users seem to find everything amusing. But once it wears off, addicts are desperate to get another fix, to the point where they are convinced that random people possess vrax, and are willing to kill to get it. Prolonged usage invariably leads to death, and vraxoin abuse has been known to destroy whole planets. In order to finally solve the vraxoin problem, officials incinerated the entire planet on which the only known source existed.
This eliminated vraxoin trade for some time, until a second source - the Mandrels - was located by a zoologist named Tryst on the planet Eden. Eden is a lush jungle planet with a red sky, and a variety of nasty indigenous life. Aside from the Mandrels, Eden is also populated by a kind of glowing insect whose bite stuns. There is also a carnivorous, people-eating plant which snags its prey with vines and then draws the prey into its maw. Biting through a root - causing its green blood to squirt out - makes it let go.
Tryst had come to Eden as part of his research to conserve all the lifeforms of the galaxy. In order to do this, he created a machine called the Continuous Event Transmitter, or CET, in which specimens were converted to electromagnetic signals and stored on event crystals in the machine, where they continued living and evolving. This process scooped up environments from their respective planets, leaving only bald patches, and turned them into laser crystal recordings which could be viewed at any time through the CET. Romana was suspicious of this process, calling it a "crude form of matter transfer by dimensional control", and was particularly alarmed by the fact that a materialization collision between the Hecate and the Empress - the ship on which the CET was being kept - had caused unstable matter interfaces which could affect the dimensional matrix of the machine. The Doctor concurred, pointing out the various things that could be wrong with the CET: its spatial integrator, transmutation oscillator, hologistic retention circuit, and especially the fact that it did not possess a dimensional osmosis damper. Because the recordings were restructured on an intradimensional matrix when played back, the lack of a dimensional osmosis damper made the display like a TARDIS, able to be entered by the people viewing it - or exited by the creatures within it.
The Mandrels were able to escape the recording through unstable barriers around the matter interfaces, from which they got into the locked ships. They began rampaging through it, killing the people they encountered, until the Doctor used his dog whistle to herd them back onto the CET and shut it down. Tryst, who had planned to use the vraxoin money to finance his expensive research, was caught in the act of trying to transfer the crystal to the Hecate using an entukor laser. He and his partner were taken into custody.
