At approximately 10 p.m. last Saturday, an olive-green presidential helicopter landed without fanfare on the south lawn of the White House, bringing Jimmy Carter home from a New England political tour.

As far as anyone knew, it was a routine homecoming for the president. Earlier, Carter had been campaigning for Sen. William D. Hathaway (D-Maine) and a string of candidates in Massachusetts. It had been a tiring day.

But instead of going upstairs to bed, Carter strode straight to a White House meeting room, where his top economic advisers were assembled, having been summoned from social engagements all over town.

By the time the meeting ended, the president had approved a plan for the most sweeping dollar-rescue effort since the Nixon administration severed the link between the dollar and gold in 1971.

Carter shattered his previous policy. But in a city ride with leaks, policymakers managed to keep the decision secret until early yesterday morning. Treasury Secretary W. Michael Blumenthal kept in touch Tuesday by pay phone from Tulsa, where he was making a speech.

The plan had its roots in the International Monetary Fund meeting of last September, where U.S. officials first sounded out their foreign counterparts on developing a contingency plan to bail out the dollar.

At the time, the dollar was continuing to decline steadily on the foreign exchange markets, though not enough to send policymakers scrambling. The plan lay in reserve until Blumenthal revived it last week.

The turning point for the administration came exactly a week ago, when the dollar began a high-speed side in the wake of Carter's announcement of his new wage-price guidelines program.

Officialsviewed the slide as irrational, way out of line with even the most pessimistic assessment of U.S. economic prospects. There were fears it could get out of hand. Blumenthal decided it was time to act.

The fitst real go-ahead from the president came after a budget briefing on Thursday afternoon. That night, Vice President Mondale and domestic adviser Stuart E. Eizenstat were let in on the plan.

But all of Carter's key political aides - including staff chief Hamilton Jordan and press secretary Jody Powell - effectively were shut out. Any hint that the action was coming would ruin the element of surprise.

The finishing touches to the plan were agreed on Saturday afternoon in a four-hour meeting of top Carter economic advisers. At the end, they phoned the president: Could they meet together later that night?

The briefing Carter received was a somber one, The situation had gotten so bad it was now time to act even if it meant risking a recession. Previously, the administration had shunned any such move.

After Carter approved the plan, the next step was to contact key foreign finance ministers and set up the international arrangements for the major economic powers to present a united front. That work went on all day Sunday.

Early Sunday morning, Blumenthal and new Carter anti-inflation czar Alfred E. Kahn took time out to appear on separate televisions interview shows - with not a mention of the secret dollar-rescue plan.

By Monday night, after the dollar had declined further, policymakers had decided they had to act quickly, with a target date of Wednesday morning. Last-minute details were ironed out. The timing of the announcement was set.

On Tuesday, Blumenthal flew to Tulsa for his speech, with no outside hint that the rescue plan was brewing. At noon, he quietly slipped out to telephone a top aide about the plan - from a pay phone. That night, he checked out early, and flew back to Washington.

Yesterday, Carter made a surprise visit to the White House press room to underscore the importance of his new actions, which resulted in rave reviews from the currency markets and some nervous fears at home.