On 8 November, Prime Minister Narendra Modi gave only four hours’ notice that virtually all the cash in the world’s seventh-largest economy would be effectively worthless.
The Indian government likes to use the technical term "demonetisation" to describe the move, which makes it sound rather dull. It isn’t. This is the economic equivalent of "shock and awe".
Do not believe reports that this is primarily about bribery or terror financing, the real target is tax evasion and the policy is very daring indeed.
You can see the effects outside every bank in the country. I am in Tamil Nadu in the south of India and here, as in every other state in the country, queues of people clutching wads of currency stretch halfway down the street.
Mr Modi’s "shock and awe" declaration meant that 1,000 and 500 rupee notes would no longer be valid.
These may be the largest denomination Indian notes but they are not high value by international standards - 1,000 rupees is only ?12. But together the two notes represent 86% of the currency in circulation.
Think of that, at a stroke 86% of the cash in India now cannot be used.
What is more, India is overwhelmingly a cash economy, with 90% of all transactions taking place that way.
And that is the target of Mr Modi’s dramatic move. Because so much business is done in cash, very few people pay tax on the money they earn.
According to figures published by the government earlier this year, in 2013 only 1% of the population paid any income tax at all.
As a result huge numbers of Indians have stashes of tax-free cash hidden away - known here as "black money".
Even the very poorest Indians have some cash savings.
But lots of Indians have much more than that.
It is not unusual for half the value of a property transaction to be paid in cash, with buyers turning up with suitcases full of 1,000 rupee notes.
The size of this shadow economy is reckoned to be as much as 20% of India’s entire GDP.
Mr Modi’s demonetisation is designed to drive black money out of the shadows.
At the moment you can exchange up to 4,000 (48 pounds) of the old rupees every day in cash for new 500 (6 pounds) and 2,000 (24 pounds) rupee notes.
There is no limit to the amount that can be deposited in bank accounts until the end of December, but the government has warned that the tax authorities will be investigating any deposits above 250,000 rupees (2,962 pounds).
Breach that limit and you will be asked to prove that you have paid tax. If you cannot, you will be charged the full amount owed, plus a fine of 200% of the tax owed. For many people that could amount to be pretty much the full value of their hidden cash.
This is brave politics. Some of the hardest hit will be the small business people and traders who are Mr Modi’s core constituency. They voted for him because they believed he was the best bet to grow the economy and improve their lot. They will not be happy if he destroys their savings.
Mr Modi says he is simply delivering on his pre-election promise to tackle corruption and tax evasion.
He says he warned that he would squeeze black money out of the system and had already offered amnesties to those who declared their black money holdings.