A helter-skelter mishmash of soapy melodrama, caper comedy and creepy thriller, "If Tomorrow Comes," a seven-hour CBS mini-series from a Sidney Sheldon book, is entertainingly loony and vice versa. Carmen Culver adapted the novel for television, and one feeling you get as you watch the film is that it must have been a lot of fun to write.

As directed by resilient old pro Jerry London, it's also a lot of fun, if foolish fun, to watch. The mini-series begins its run tomorrow night at 8:30 after President Reagan's message on Central America and continues Monday and Tuesday nights at 9, on Channel 9. It's the kind of show that if you believed a word of it, it might ruin the whole thing. But not to worry.

Madolyn Smith stars as Tracy Whitney, a young woman who, in the first hour, suffers through a streak of riotously bad luck. Engaged to a rich drip whom she nonetheless really loves, she returns to her native New Orleans upon learning of her mother's suicide there. One thing insists on leading to another, and before you can say "Lillian Gish" she is up the river on a trumped-up charge of attempted murder, being terrorized by lecherous caged lesbians.

One of these, the very single-minded Bertha, is played by Susan Tyrell, an actress who will stop at nothing not to be boring. With the help of a sympathetic and soon-to-be-sprung convict (C.C.H. Pounder) and her own resourcefulness at rescuing the warden's drowning child, Tracy is released into a world that is just waiting to clobber her once more.

Since "respectable" society will now have nothing to do with her, she falls into an inventive life of crime. The way she fools the cops when completing her first housebreaking assignment is masterful. The mini-series romps on from there.

Director London is able to give hokum like this a daft energy that makes matters of credibility irrelevant. It's clear he's possessed of a certain wry esprit when he puts the prefatory suicide just prior to his own name in the opening credits.

In the first chapter, London juggles two parallel story lines: Tracy Whitney and her eventful travails, and the adventures of con man Jeff Stevens, played with roguish brio by aging but agile Tom Berenger. The two paths cross once on a yacht; then we have to wait around through oodles of trauma before the two of them team up for a life of fleecing the rich who deserve a fleecing.

Corruption seems to be the theme of the piece. The con man at first tries to go straight and marry an heiress (that flinty-sexy Susan Hess again, fresh from "Dress Gray" duty), but she cheats on him and he discovers that the Wall Street business types of her privileged class are merely con men in tailored suits, filching and chiseling where they may.

He returns to the con-person life style (encouraged by the unfailingly good Jack Weston as porky Uncle Willie), and a world in which thieving jewelers, conniving movie producers and myriad other mink-collar crooks are ripe for plucking.

Like any good escapist trash, which this is, "Tomorrow" hops around Europe and eventually settles in Cannes and Amsterdam. The filmmakers are clever enough so that you don't feel you are watching clothing and settings occasionally obstructed by actors. The story is bilge, but it's captivating bilge. You want these charming crooks to get away with their loot and, it is spoiling no suspense to say, they do.

Smith wears well over the three nights, though her idea of a come-hither look is on the go-thither side. Tracy and Jeff don't share a thieves' bed until the seventh hour, which is a little late, and there is too much cat burglar and mouse burglar spatting once they team up for joint ventures. Also, Culver never includes what should have been an obligatory scene in which Jeff would say to Tracy, "Tell me the story of your life," and he would realize how their fates had been earlier linked. It's an awfully clumsy omission for such a proficient lark.

Richard Kiley has a high time with the role of Gunther Hartog, master thief, but the showiest performance is that of David Keith as an obsessive insurance investigator who is also, in his spare time, a homicidal maniac. He sits around watching matches burn and haunting cathedrals. Considering the fact that London and Culver make a big deal out of menacing portents involving Keith and his matricidal flashbacks, it's too bad the inevitable showdown near the film's end is so matter-of-factly brushed aside.

You wonder why they went to all the trouble of establishing Keith as a plot element and then sort of decided to forget the whole thing. It would have been funnier, really, if Tyrell had escaped from prison and taken it upon herself to chase Smith all over the globe. But Keith makes the most of a no-win situation.

The idea that there is an international subculture of con artistes who prey only on the over-rich is a pleasant, perhaps even cheering, conceit. Of course all this would have been still more satisfying if one of the victims were a gauche old goat who'd made millions writing trashy novels that were then turned into frivolous network mini-series. But, you can't have everything.

'W.C. Fields Straight Up'

Public TV documentaries mining lodes of Hollywood nostalgia appear to have become a genre, one in greatest evidence at fund-raising time. Thus it is that as quixotic a figure as W.C. Fields is called in to the service of PBS tonight with the broadcast of "W.C. Fields Straight Up," a rampage of a remembrance at 10 on Channel 26.

Fairly standard in form but rich in content, the 90-minute film, which was directed by movie buff Joe Adamson and coproduced by Fields' grandson Ronald, surveys the Fields career from his earliest vaudeville juggling days through his fractious Paramount features and on to his glory days at Universal, where he made "The Bank Dick," "Never Give a Sucker an Even Break" and "The Man on the Flying Trapeze."

A teetering monument to incorrigibility, Fields developed into a magnificent mock-heroic figure on the screen, battling the forces of gravity, bratty children and belittling wives. He was virtually alone in the dark-comic tone he brought to his lunatic feature pictures; like few other comic actors ever on the screen (Jerry Lewis and Woody Allen, for example), Fields was an auteur who earned a large measure of control over his films.

Whether landing in Wuhu and insisting it to be Kansas City; religiously cheating at cards, pool and golf; kicking a disreputable tot squarely in the pants; chasing an insurance man with a meat cleaver; setting a summons on fire; or lassoing an ostrich on the street, Fields was a man forever at odds. And the odds were always against him. His comedy was the philosophical statement of a sentimental misanthrope.

Surviving colleagues talk about the man and his drinking problem, but not about his mistress, a story already told elsewhere. "He drank as much as they say he drank," says grandson Ronald Fields, but some of the other legends about Fields, narrator Dudley Moore points out, are untrue, yet will probably endure. Paramount newsreel footage of an earthquake occurring during the making of a Fields picture turns out now to have been a collaborative fake by Fields and director Edward Sutherland.

"He admired anybody who could give him a good fight," Edgar Bergen is heard to say in an old audio recording. Gloria Jean, a child costar of Fields, remembers that Fields was warned never to drink in front of her on the set. Others reminiscing include Joseph L. Mankiewicz, Madge Kennedy (who played with Fields in the stage version of "Poppy," later filmed) and Will Fowler, a longtime friend.

A man of many names -- Egbert Souse', Mahatma Kane Jeeves, Otis Cribblecobbis, Larsen E. Whipsnade -- Fields' life and career seemed founded on the notion that in a world ruled by madness, the best course is to take a drink and bluff your way through. He took too many drinks of course, and the last one killed him, in 1946. But he was still a helluva Galahad.

It's a pity that Adamson and the producers didn't identify more of the actresses who played foil to Fields in the film clips included (Margaret Dumont and the truly formidable Kathleen Howard among them), and that they didn't include, near the conclusion, Fields' touching goodbye to Freddie Bartholomew in "David Copperfield," which is cited as the only Fields film in which he recited the lines in the script as they were written.

But this is still a touching and trenchant tribute to a singular comic force. An old poster from Fields' days as "the greatest of eccentric jugglers" declares, "He makes you laugh even whilst wondering at his deftness." It is as true now as it was then.