The tourney at Ashford Meadow
A hedge knight of enormous size and no particular breeding buries his old master, calls himself Ser Duncan the Tall, and rides for a tourney with a boy named Egg holding his horse. What follows ÔÇö a broken puppeteer's hand, an accusation against a prince, and a trial of seven ÔÇö is the beginning of the friendship that quietly determines the shape of the next century.
The fuller tale
An old hedge knight died in a ditch on the way to a tourney, and his squire buried him, took his horse and his arms, called himself Ser Duncan the Tall, and went on to the tourney anyway because there was nowhere else to go. On the way he picked up a bald boy who wanted to hold his horse and would not go away. The boy was Aegon Targaryen, fourth son of the heir to the throne, and he had run off from his family to see the world.
At Ashford Meadow, Dunk saw a prince of the realm beat a puppeteer for the crime of having a puppet show with a dragon in it, and did the thing that every rule of his position told him not to do: he stopped it, and broke the prince's hand doing it. Striking a prince costs a commoner his hand and his foot.
He demanded trial by combat instead, and because his accuser was a prince it became a trial of seven ÔÇö seven champions a side, the old form, not used in living memory. Three of the seven who rode for him did so because Egg asked. Baelor Breakspear, Prince of Dragonstone and heir to the Iron Throne, rode for a hedge knight against his own brother's son and was killed by accident in the doing of it, by a blow struck by his own son.
A very small door
Everything afterward turns on this. Baelor Breakspear would have been an excellent king and did not become one. The succession slid sideways to men who were not equipped for it. And the boy holding the horse ended up on the throne himself, decades later, with a hedge knight for a friend and a set of opinions about the smallfolk that he could only have got from walking among them in dusty boots.
Ashford is the smallest event in this chronicle with the largest consequences, and it happened because one large slow man could not watch a puppeteer be beaten.
Elsewhere in the wiki
 See the world as it stood in 209 AC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.