Sisyphus was an organic computer created by an ancient race of people. At some point it began to harvest all living creatures on its homeworld by plugging them into its mainframe and harvesting the organic matter from their brains to sustain itself. It did this by putting its victims with a simulated world in which they were presented with an idyllic new life and were gradually convinced that their old life had been some sort of delusion. Once its victims fully rejected their old life and adjusted to their new life, the organic matter in their brain was harvested until nothing left of their mind remained. Once it exhausted all organic matter on the planet, it sent out a fake distress signal in order to lure in passing travellers.
When it harvested Leela and Jenny, it placed them in a scenario where they were scientists in 1978 London working on something called the Crowmarsh Experiment. In this experiment, Sisyphus was the name of a drug used to induce dreams so vivid that the subjects' minds create a whole new identity for themselves based around this dream. It is this framing that is used to try and convince both Leela and Jenny that their old lives were a fiction and that Sisyphus's mainframe is the real world. The Doctor is able to tap into the mainframe and help both Leela and Jenny escape by instructing them on how to destroy the Crowmarsh installation. Despite Sisyphus's attempts to thwart them, Leela is successful in setting fire to the building's gas mains, forcing her to wake up, but not before Jenny's mind is harvested. Before Jenny's mind is completely erased, however, she leaves a message to override the distress signal at the Doctor's behest, warning other travellers to stay way from Sisyphus' homeworld. (AUDIO: The Crowmarsh Experiment)