The Democratic National Committee had offices in the Watergate complex - which held both offices and hotel rooms. During Nixon's second campaign for President, those offices were burglarized in an attempt to ascertain the campaign strategy for Nixon's opponent. The burglars are caught during that attempt.

A tip was given to Bob Woodward that the burglary was committed by allies of President Nixon. This tip came from a man calling himself "Deep Throat" - later revealed to be FBI Associate Director Felt.

As the investigation into that burglary continued, it was discovered that not only were the Republicans behind the burglary, but Nixon himself had authorized it. This was discovered because Nixon secretly recorded many of the interactions that he had in the Oval Office, including discussions of the break-in among them.

Nixon knew that this was damning evidence of a criminal conspiracy, and ordered that the special prosecutor be fired, leading to the resignation of key officials in the administration.

When the SCOTUS ruled that Nixon had to hand over the tapes, it was basically the end of his term. Articles of Impeachment were being drafted, and Nixon was told that he didn't have the votes in Congress to survive. On Aug 8th, 1974, Nixon resigned. That September, Ford pardoned Nixon for all his crimes.

A full timeline is here: https://en.wikipedia.org/wiki/Timeline_of_the_Watergate_scandal