Along with Stones of Blood, Ribos Operation is the best from the Key to Time Season/series. An all-round enjoyable story with a constantly shouting bad guy, two unorthodox con-men who adopt strange northern accents, and the good Doctor and his new companion Romana.
Although the story is not especially original, the script is fairly tight, and everything seems to work. Perhaps by the fourth episode it seems slightly padded, but there's always enough going on to keep you interested.
Most valuable player in this story must go jointly to the two conmen, who are of the old-school, honorable type. In other words, they'll switch the precious stone from the doctor for a piece of rock, but when he switches it back and they find out, they are appalled!!
It's good fun.