One of Robert Louis Stevenson's least-read novels about 18th-century nobility has been transposed into an engrossing 20th-century television entertainment. The Master of Ballantrae (CBS, Tuesday, Jan. 31, 8-11 p.m.) is a blowzy family saga, impossibly contrived and often implausible, but always fascinating because of the outrageous machinations of its principals. ''Master,'' filmed on location in Great Britain by cinematographer Bob Edwards, is tastefully gorgeous when it must be, but tastelessly gaudy when the plot calls for it.
Richard Thomas, in a complex part which calls for naivete which evolves into evil madness and then contrition, manages to make a basically unbelievable character believable. Mr. Thomas, who has played a variety of roles in recent years, is proving himself to be one of our most dependably excellent actors. Michael York, as his totally evil brother, is so vile and nasty an incarnation of the devil that you may find yourself hissing at the TV screen.
John Gielgud gives another of his charming old-timer characterizations, more wry and lovable than ever. Oh, to see him as a villain one of these days!
The plot of ''Master'' revolves upon that old conceit - a good brother and a bad brother, a family estate, a woman whom both love. Bad brother James (York) is constantly being reported dead but turning up alive, while good brother Henry (Thomas) suffers in silence until he, too, turns evil in a mixture of frustration and desperation.
The story takes us to British locations and studios decorated to represent Canada, New York, and India. It ends on a subtle note of indirect triumph for evil - in Scotland. You won't believe a word of the plot, but it's a lot of fun to watch the actors try to make the hokum convincing.
''Master of Ballantrae,'' directed skillfully by Douglas Hickox, is a presentation of the always tasteful ''Hallmark Hall of Fame'' series. So the whole family can get the corn popping and curl up for this electronic equivalent of a ''good read.''