The mansion contains many interesting rooms and seven other people: Tom, a plumber; Sam, a mechanic; Sally, a seamstress; Dr. Green, a surgeon; Joe, a grave-digger; Bill, a butcher; Daisy, a cook.

At the end of the 1970s, Ken Williams sought to set up a company for enterprise software for the market-dominating Apple II computer.

Roberta decided that she could write her own, and conceived of the plot for Mystery House, taking inspiration from Agatha Christie's novel And Then There Were None.

Ken found, however, that the resulting digital drawings were too large to fit into a 5¼-inch floppy disk, so he devised a way to convert the images into coordinates and instructions for the program to redraw the lines of the scenes rather than static images, as well as writing a better version of the VersaWriter scanning software.

The couple took out an advertisement in Micro magazine as On-Line Systems, and mass-produced Ziploc bags containing a floppy disk and a sheet of instructions, to be sold at US$24.95 (equivalent to $95.22 in 2024).

[4][7] To the Williamses' surprise, what Roberta had initially considered a hobby project sold more than 10,000 copies through mail-order.

[8][4][9] Including its 1982 rerelease through the SierraVenture line, 80,000 units were eventually sold worldwide,[10] making it one of the best-selling computer games at the time.

In 1982, MicroCabin released Mystery House, which was unrelated to (but inspired by) the On-Line Systems game of the same name.