There was an equivalent of the Plunge Protection Team around during the 1929 October stock market crash. It was formed by the big banks under the leadership of JP Morgan. On one of the crash days this syndicate purchased large amounts of key stocks at prices well above well above the offering price in an effort to halt the collapse. And they were successful, at least on that day. But the crash resumed the following day and the bankers stepped aside, realizing that they were simply throwing good money after bad.
The ‘Plunge Protection Team’ of today is much more powerful than its banking counterparts of the late 1920s, because it comprises the Federal Reserve, the SEC and the US Treasury. This entity has the wherewithal to throw huge amounts of money into the market. But the stock markets are governed by natural law and under the circumstances no amount of money can turn a bear into a bull.