Don eventually finds out from Barney and Lily about the past relationships and, unable to comprehend that Robin continues to hang out with two of her exes, leaves the bar.
Ted reads his own letter; after he mentions Robin's then-fear of commitment, he realizes he too misses her.
They wind up crashing Don's place, much to Robin's embarrassment, especially as Ted has re-stolen the blue French horn for her.
The next day, a hungover Ted and Barney apologize, but Robin reveals she wants to take a break from the group and has agreed to move in with Don.
[3] Alan Sepinwall of Uproxx compared it to The King of Queens episode "Bed Spread", which he said was even more funny.