Picture this scenario: Mid-September in New England; a high-pressure area sweeps through, bringing fair skies; then the front stalls near Bermuda.

The result in the waters of Rhode Island Sound is a protracted period of mild, gentle weather, a September season New Englanders call Indian Summer. The breeze comes from the south, a whisper, never topping 10 knots.

Perfect weather for larceny -- the grand theft of the most cherished jewel in yachting, the America's Cup.

A month ago, seven yachts and 77 yachtsmen were chasing the Cup. The August knife cut deep and now only two boats remain -- the sleek blue American 12-meter Freedom and an aluminum boat from the other side of the world called Australia. On Tuesday they begin racing for the Cup in a best-of-seven final series off Newport, R.I.

Foreign challengers have pursued the Cup since 1851 but have yet to win it. Traditionally the most exciting America's Cup competition has been between U.S. yachts vying for the right to defend.

No one expected this year to be different. Australia was not significantly changed since her 4-0 defeat by Courageous in the 1977 Cup final. Meantime, the Americans had advanced skills and technology with Freedom, which defeated Courageous every race but one this summer during trials to select a U.S. defender.

But while Australia, under veteran skipper Jim Hardy, was dispatching challengers from France, Sweden and England on the high seas, the Australian syndicate's shakers and movers were building a secret weapon with which they intend to snatch the silver chalice from the unsuspecting Americans.

Now the secret weapon is in place.

A week ago, the day after the Australians whipped Baron Marcel Bich's France 3 and won the right to race Freedom, the Australian contingent watched proudly as a new mast was stepped aboard the big snowy yacht.

The mast had been built in secrecy in a deserted warehouse on the Newport waterfront.

It was standard size but the top 20 feet of aluminum had been lopped off and replaced with a fiberglass substance that bends about twice as much as aluminum. The bendy mast, invented this year by the British, creates a different sail shape that allows a 12-meter to carry about 200 square feet more mainsail than a conventional rig.

In light airs, the British had proven, the advantage can make a boat unbeatable.

Now the Australians are hoping for the right weather. They're taking a long shot, but it beats practically no shot at all.

Traditional mid- and late-September weather patterns in New England find a series of cold fronts passing through. The frontal patterns bring north winds of generally 15 knots or better.

But "it's not impossible for a high to stall over Bermuda this time of year, producing an extended period of light southerly breezes, according to Gerry Rigdon of the National Weather Service.

The Australians have an interesting option, as well. They claim to have worked out a system of switching masts so that in two hours, depending on weather prospects, they can shift between the bendy setup and their old conventional rig. Said one source, "It's certainly possible that we might switch the rigs overnight."

There are a few small racing sailboats that offer a choice of big or small sail rig, based on wind strength, but never before has it been tried on anything like the scale of the massive 90-foot-tall, 12-meter rig. If it works, it could change the nature of 12-meter racing, with interchangeable spars and sails a requirement for a competitive boat.

But it's risky business at this late stage. "Sure it's risky," said Warren Jones, the Australian project manager. "But the saying is, 'He who dares, wins.' We just may be the right people to dare."

Australia is not exactly battling the giant alone. Jones said that last week he had help from all three of the foreign camps his crew beat for the right to challenge. France 3 provided boat and crew as a trial horse for Australia to warm up against; the Aussies were using the Swedes' slip at Newport Offshore yacht yard and an English crewman had donned frogman gear to help with some underwater hull work. It marked unprecedented cooperation among rival challenging camps.

Over in Freedom's camp the mood is optimistic, though perhaps a bit more cautious than it might be for a boat and crew that have wiped the floor with every bit of competition that has been thrown at them all summer long.

"There's no way to estimate now what she (Australia) will do," said John Marshall, president of North Sails and Freedom's sail tacticican. "I'd give the edge to Freedom on sails, tactics and boat handling. We've had about 60 races this summer alone, compared to 15 or so for Australia.

"My feeling is this: If the racing is close, Freedom enjoys a substantial edge because of her sails and tactics. But if that mast makes a big difference and Australia runs away from us, there's not much we can do . . ."

Marshall said Freedom skipper Dennis Conner and others in the American defender's camp had a strong feeling for the last month or so that the Australians were building a bendy mast.

Why then didn't this best-prepared-ever U.S. defender gear up and build its own bendy mast, to cover all options?

"We toyed with that notion," Marshall said, "and decided that we stood to lose more than we'd be likely to gain." The Freedom camp spent 18 months and more than $2 million building an organization Cup watchers have come to call "the machine." They didn't want to mess with it at the last minute. "We had to consider the very real possibility of a severe setback if the bendy mast didn't work," said Marshall.

"We've done everything we can to prepare for this match," he added. "Now it's out of our control. It's reasonable to say that racing will be very tough if the wind is light and their mast is squared away.

"But it's still seven races in New England in late September and if conditions are what they should be and they either can't use the new spar or do use it in the wrong conditions, we're going to beat them."