The PBS Mystery series goes full-tilt Gothic in an unusual one-night, 2 1/2 -hour movie that will be on Maryland Public Television, Channels 22 and 67, tonight at 9 o'clock. Peter O'Toole is the big name in "Dark Angel," but the star is young Beatie Edney as Maud, a 19th century teen-ager who has led a life of virtual isolation on her eccentric father's palatial estate. It's the flip side of Jane Austen country, where the long distances between the moneyed types do not breed a well-mannered naivete, but instead allow evil to fester unnoticed, and unchecked. And when it's a woman who's not yet of legal age who finally does notice and put a stop to it, no one is going to believe her over the word of a gentleman who at least used to be of some means. That would be O'Toole's Silas, Maud's uncle. Growing up, Uncle Silas had been a mysterious character in Maud's family, mentioned only in hushed tones, surrounded by some unknown scandal, which turns out to be that he was suspected of involvement in the apparent suicide of a wealthy gambler who was found dead in Silas' rambling mansion. Seduced by Silas' image as a --ing young man painted in an impossibly-Baroque portrait that hung in her home, Maud had always been attracted to her mysterious uncle. So, when her father dies and his will states that she inherits everything, but is to live with Silas until she comes of age, she agrees immediately to that stipulation, despite the misgivings of some other family members. Her widower father had hardly cared for Maud properly, pretty much ignoring her, but he had provided a benign enough environment. However, his hiring of a sadistic French governess for her, one Madame de la Groviere, played by Jane LaPotaire, who turns out to be the story's bad penny, is a foreshadowing of what's to come. Maud assumes that a similar life -- a bit more exciting if only because it contains an element of unpredictability -- awaits her at her uncle's mansion. But, when she gets to Silas' decrepit estate, she finds not the handsome Lawrence of Arabia who stared back from the portrait, but O'Toole's gaunt, almost cadaverous, visage. What the years, and huge amounts of substance abuse, have done to Silas' -- and indeed O'Toole's -- face turns out to be an appropriate analogy for what has occurred to this once-proud place. The physical decrepitude of the place mirrors the state of the spirits of most of those who inhabit it; the better parts of their human souls have long since fallen into disrepair. Soon enough, it is a dark and stormy night. Peter Hammond's direction of this BBC production is filled with gimmicks, but services the melodramatic aspects of the story quite well. He is aided by some spectacular locations, spooky-looking manses that must dot the countryside in England. Edney brings an appropriate fresh-faced beauty to the role of Maud, playing her as naive, but still tough and resourceful. LaPotaire's Madame de la Groviere is a compendium of every stereotype of French that the English love to believe. But it is, of course, O'Toole who steals the show. This guy doesn't have to chew the scenery; he can devour entire sets with the smallest nibble. Just catch his roll of the eyes as he partakes of one of his nefarious potions to realize how O'Toole understands a man who has given way to the darkest angels of his psyche. Usually, Mystery runs its shows an hour-long episode at a time, meaning that "Dark Angel," with some previews, recaps and introductions by host Diana Rigg, could have been shown over three weeks. The decision to go with one night serves it well. The plot's confusing enough as it is -- indeed, the show could use a "Perry Mason"-type scene at the end in which a Paul Drake shows up and explains everything -- but the real appeal of "Dark Angel" lies in its ability to create a mood of mystery and suspense. It makes for a dark and stormy night. * The Maryland Film Commission is once again looking for participants for a comedy writer's workshop. Last year, 17 local writers spent a week honing their skills with representatives from Warner Bros. Reflecting their recent partnership, Lorimar Television is participating this year. The rules have changed a bit. Only scripts for existing half-hour situation comedies, written by residents of Maryland, Delaware, Virginia and Washington, are acceptable. They must be submitted to the commission by April 2. Entry forms can be obtained by sending a self-addressed, stamped envelope to the Maryland Film Commission, 217 E. Redwood St., Baltimore, Md., 21202, or by visiting those offices. For further information, call (301) 333-6632.