The Rainbow was an intergalactic gaming cartel. According to the Sixth Doctor, they were "vicious" and "amoral".
Mel was forced to play a game against the Rainbow and used the computer room where Guy Oliver was doing surveillance in Hong Kong. At stake was the 20th century — if Mel lost, the cartel would take Earth out of time and sell its technological development. With the Doctor's help, Mel not only defeated the cartel but sent their identities to the authorities. (PROSE: Old Boys)