Come for a ride in The Runaway Elevator, a thrill and a chill in a turn-of-the-century iron cage.

Up, up, up. Then suddenly . . .

"Near the top," the script says, "the cable jams. Rescue voices filter down from overhead. The escape hatch opens and just as we are about to be rescued a creak and groan come from outside the elevator cab. The next shudder signals the inevitable . . .

"Through the open hatch," the script goes on, "the guests can see that the elevator cab is hurling uncontrolled down past floor after floor."

But don't lose hope. "Fortunately this scene has a happy ending. The cab shudders to a stop just before impact."

This high-riding adventure is a sample of the latest extravaganza scheduled for Baltimore's much-celebrated Inner Harbor.

On a waterfront pier, a $19.8 million array of indoor games, rides, shows and other hoopla is being planned--with official city blessings--by the Six Flags Corp., a Los Angeles-based amusement park outfit. The offerings are expected to include a Ferris wheel, pinball, dancing clowns, a gourmet restaurant, a carousel and such walk-through fantasies as Dr. Fanto's Incredible Electric Time Machine, a maze-like escapade in a make-believe submarine, dirigible, locomotive and moon rocket.

The aim is to "bombard the senses," says Laurence B. Miller, Six Flags' marketing director. He terms the scheme a new form of urban entertainment. "You won't find anything like it in any other city."

Novel, too, is the setting. The urban amusement park will be packed inside a long-abandoned, turn-of-the-century electric power plant. The cavernous brick edifice, a complex of three interconnected buildings erected between 1895 and 1902, has been praised as an example of late 19th century industrial architecture, marked by arched windows, tile walls and ornamental masonry. It also has been described less charitably as an outmoded facility, filled with coal dust and pigeon droppings.

"We cleaned it of pigeons a few months ago. So it's in decent shape," M. Jay Brodie, Baltimore's housing and community development commissioner, was quick to point out. "We're rather excited about it. . . . It's quite a magnificent building."

The pride Brodie and other Baltimore officials have taken in the new venture partly reflects the old plant's site. It is situated on a leading edge of the inner harbor area just east of the city's 1 1/2-year-old National Aquarium and beneath the towering World Trade Center. The inner harbor is Baltimore's urban redevelopment showcase, a blend of waterfront tourist attractions and new office buildings.

The Runaway Elevator and other episodes in the project's fanciful scenario are being dreamed up by a Six Flags team that includes science fiction writer Ray Bradbury, author of such tales as "Fahrenheit 451" and "The Electric Grandmother."

"We have so many ideas that we now have to weed them out," Bradbury said in an interview. The group's strategy, he added, is to ask, "If we had a giant toy box to cram full of ideas, what would be fun?"

Six Flags promises that the indoor amusement complex, expected to open in the summer of 1984, will display a turn-of-the-century theme, befitting the building's historic past. The facade is to be restored and spruced up. Some of the old boilers, hoppers and other equipment are to be preserved. The entertainment will mix Edwardian charm with Industrial Revolution technology.

In a 1905 article, Scientific American hailed the coal-fired plant as representing "the highest development of power station engineering" in its time. The plant, the journal said, "had the distinction of producing current at the lowest price per kilowatt hour of any station in America."

In its earliest years, the generating plant supplied electricty for street car lines operated by the City and Suburban Railway Co. and its successor, the United Railways and Electric Co. The plant, known as the Pratt Street Station, was taken over in 1921 by the Consolidated Gas, Electric, Light and Power Co. of Baltimore--Baltimore Gas and Electric Co.'s predecessor--and was converted in 1929 from generating electricity to supplying steam for heating and other uses.

On Jan. 1, 1973, the plant was shut down, chiefly because of new air pollution regulations requiring costly modifications for coal-fired facilities. The city bought the abandoned structure from Baltimore Gas and Electric for $1.65 million in mid-1977 and embarked on a trouble-crossed search for a suitable tenant.

The first deal, a plan for a $55 million luxury hotel, fell through. The city had reached a tentative pact in 1979 with Mortimer B. Zuckerman, chairman of Boston Properties and owner of The Atlantic magazine, for the 365-room project. "The problem was that it cost too much," recalled Martin Millspaugh, president of Charles Center-Inner Harbor Management Inc., a nonprofit firm that advises the city. After 1 1/2 years of negotiations, he said, a gap of about $8 million remained. Zuckerman blames the city for not coming up with enough federal money.

Last April, officials started soliciting tenants again. In addition to the Six Flags scheme, the city was offered a proposal to turn the power plant into a winery, two plans for marine shopping centers that would have sold anything from yachts to sailing caps, and two other entertainment propositions. The city picked Six Flags shortly after Christmas and expects to cement the deal within a few months by negotiating a lease.

Today, the dark and grimy power plant conjures up some of the magic of its former era. "Amazing, isn't it? Almost overwhelming," said Jeff Middlebrooks, planning director of Charles Center-Inner Harbor Management, during a tour of the building, whose enormous interior reaches up to 123 feet in height. He climbed a spiral staircase for a bird's-eye view of the massive hoppers, brick-clad boilers and ancient coal and ash conveyors.

"This monstrous concoction of machinery is where the furnaces were," he said. "You can imagine what this was like when it was burning coal like crazy--incredibly dirty, hot and noisy."

Six Flags, a subsidiary of Bally Manufacturing Corp., a Chicago-based pinball, slot machine, video game and gambling casino enterprise, bills itself as the nation's largest operator of themed amusement parks, a leisure-industry pacesetter with $286 million in revenues in 1981. Its six parks include Six Flags Over Texas, Six Flags Over Georgia and Great Adventure in Jackson, N.J. The company also owns two wax museums and the Tom Foolery restaurant chain.

In Baltimore, Six Flags officials predict, the new spectacle will attract about 1.2 million visitors a year. Though it initially sought government financial aid, the company eventually agreed to finance the project itself. Its preliminary proposal forecast annual revenues of $18.5 million and before-tax operating profits of $5.9 million. For Baltimore's economy, the venture is expected to provide 200 to 300 new jobs and help generate millions of dollars in tourist spending.

Though the script for Baltimore's Six Flags Power Plant is still being written, the company's proposal, submitted last July, offers a glimpse of its tentative scheme. White steam puffs will rise from the refurbished plant's four tall smokestacks. A late-afternoon whistle will sound. Multicolored balloons will float skyward from its roof.

A basement museum, with free admittance, will explore turn-of-the-century power plant technology. An elegant Crystal Pavilion will offer gourmet dining in a lace-like structure of glass and wrought iron. An Edwardian Music Hall will feature an oyster bar along with daytime and evening entertainment. There will be an ice cream and soda fountain. A few shops, run by Six Flags, will sell souvenirs, toys, costumes and photos.

The building will be immersed in a confection of animated films, Broadway-style musical shows and assorted happenings, many of them available for a single admission price of $7.95 for adults and $5.95 for children.

A Victorian Balloon Ride will give tourists a vivid impression of what it would be like to fly over Baltimore's historic sites. There may also be a Victorian Swing Room, where visitors would ride on a large swing pushed by an attendant.

"Before long, the swing is out of control," says the tentative script. Visitors, it adds, may "fear the worst" as the swing starts to make a full revolution. But, Six Flags officials say, the swing ride would surely have a happy ending.