Officials from the US, Japan, West Germany, France and the UK met at the Plaza Hotel in September 1985 after the dollar had risen for several years and US manufacturers were lobbying loudly for relief. They agreed to act together to bring it down, and did so through central-bank-intervention and supporting policy.
The dollar fell sharply against the yen and the mark over the following two years, by far more than intended. A follow-up meeting in 1987 produced the Louvre Accord, aimed at stopping the decline the first agreement had started.
It is cited on both sides of the intervention argument: proof that coordinated official action can work when it pushes with the fundamentals, and a warning that the size of the response is not controllable.
Example: USD/JPY traded near 240 before the meeting and was near 150 within about two years, a fall of roughly 38% in the dollar against the yen.
Related: central-bank-intervention, currency-war, ninja, verbal-intervention