Martin Mull was going great guns on "Mary Hartman, Mary Hartman" until a Christmas tree got him, right in the closet. Thus did expire the character of Garth Gimble, a despicable cad whom Mull played with frightening credulity.

Our story would end there, except that Mull will be returning to the Hartman show next month in the role of Barth, twin brother of Garth, and thus become the first actor in the history of "Mary Hartman" to return after his own death.

Our story would end there, except that Martin Mull was in Washington this week appearing to packed houses at the Cellar Door - make that packed people at the Cellar Door - and shooting his mouth off at a ritzy downtown restauran that was out of London broil and out of green beans to boot.

Mull, who is 33and one of the most underfinably original and irreverent comic minds at loose in the world, and who looks and dresses not so much like one of the most undefinably original and irreverent comic minds at loose in the world as he does a computer progammer who is petending to be a CIA agent pretending to be a computer programmer, explained how Norman Lear decided to re-enlist him for "Mary Hartman."

"I begged him and begged him, and he finally said okay," said Mull, dropping his cigarette on the white tanle cloth for about the fifth time.

The vile, contemptible and hilarious Garth was not only deep into male chauvinism but a wife-beater besides. The original plan was for him to have a four-week life span on the daily syndicated soap spoof. This was extended to 13 weeks and would have been extended to 26 except that NBC had an option on Mull and was trying to develop a series for him.

They didn't, and Mull will return to the hills of Fernwood.

To Mull, "Fernwood is a real place" that exists in the Sunset Boulevard studios where "Mary Hartman" is taped. he denied that people in the cast are stoned all the time, whatever some viewers might think. "Nobody is ever stoned at all - at work.Their private lives are their own business. Some guy may go home and re-balm his grandmother and tie her to the refrigerator for all I know."

He actually smiles at this disgusting thought.

Mull speaks highly of Lear ("I refer to him as 'Rent'"), who gave him his first acting job on television, and of costar Louise Lasser. On one show, Garth had locked his wife in the closet for days, and was about to leave for work. "Iw was Louise who said, 'Martin, you should kiss the closet door good-bye,'" said Mull. "Of course she was right. I was embarrassed that I hasn't thought of it myself."

Otherwise, though, Martin Mull's Adventures in Televisioneand have not been all that satisfying. NBC has given him "a sizeable amount of money" to develop a series, but they don't seem to realize his wavelength. "They keep coming to me with things like, 'Look, you're a dentist in a small town and you have two kids, and one morning you get up and your car won't start, and the mayor is coming over for recaps!'

"You know - 'It's zany.' And it's s - - -."

Network people caught Mull's club act, saw and heard a ful house laughting, screaming, doing everything but naming a dish at McDonald's after him, but then when they get back to their network offices, they tell Mull, "Of course, those 400 people in the club were not normal people." Networks have their own ideas about who the normal people are.

Mull has his own ideas for a series: "Frontier Gyno,' the story of the first ladies' doctor in the West," for "Elizabeth and Andy," in which 'the Queen of England remarries, to a ghetto black, and they're known as Queenfish and Kingfish." He also has high hopes for "Gourmet Bowling" and for "Tony Orlando and Dawn and The Man."

"A Dadaesque attitude toward life," says Martin Mull, "is the best attitude to have."

To get exposure in show business, he has done the talk shows, including a grueling six times with Merv (Griffin, of course). But the last time he was on the show, performing his song "Do the Nothing," it began to get to him. "The audience at that show is old people and their parents," he said. "You know - shopping bags with glasses. I was out on Vine Street before the show trying to find a little screw for my sunglasses, and a guy came up to me four different times offering me tickets for the Griffin show, I finally said, "No thanks, I'm on it.'"

Mull's star is - how shall we put it - rising. He just finished making the movie "Oh, God" with John Denver and that other singer, George Burns. And Lear has asked him to direct episodes of the upcoming "All that Glitters," a new supersoap that will deal in "role reversals," with men in all the life and career roles previously filled by women, and vice versa (it will not be a transvestite follies as has been erroneously reported elsewhere).

In the meantime he will keep apparing live in clubs singing such balads as "I'm Everyone I've Ever Loved" because of "a certain amount of ham." And he will continue to be disgusted by "anyone who says what they do is heavy," by "the fact that it takes so long to get off an airplane after it's landed," and by "people who cut cartoons out of newspapers and pin them to the wall and write 'AL' with an arrow pointing to one of the characters."

So there you have it: Martin Mull. Yes sir, quite a guy. A bit of the rogue, a man with a load of mischief and a sense that the world is mad and plastic pink flamingos stuck in the potted plants that decorate his apartment in Sherman Oaks, Calif. Our story would end here, and in fact does.