The Guide Mk. II Construct was a form of artificial reality running for 100 virtual years, encasing Arthur Dent, Ford Prefect, Trillian Astra, and Random Dent.
As the world was about to be destroyed at the end of Mostly Harmless, Random Dent expressed regret at the life she never got to lead. Because she was the Guide Mk. II's secondary master, the Guide encased Random, along with her father, mother, and friend, in a virtual reality, each separate from the other. In Arthur's reality, he was at a peaceful beach, indulging in tea. In Ford's, Ford was at a luxurious spa/hotel, able to acquire all manner of food and other services free of charge, and able to drink huge amounts of alcohol yet coming out of it unscathed the next morning. In Trillian's reality, she had become a famous reporter, feared by celebrities everywhere. Random's consisted of her becoming President of the Galaxy, solving all problems with ease and uniting the galaxy. Every 20 years, the constructs of Trillian and Random merged, thus allowing them to see each other. It was so complete that the occupants thought the construct was real, thinking they'd been rescued from Earth. (Each person saw their rescue in a different way; Trillian thought their Babel fish had transported them to Milliways, and Random thought a squad of unicorns had saved them, led by Sparkle Gem True Hoof, her pen pal.)