The Klingons descend upon DS9 in an effort to thwart the Dominion threat.
This is a very memorable double episode.
It has everything in terms of entertainment, starting with a compelling plot full of political intrigue and big moments. The strategic ambition of the Klingons, in their dealings with the Federation and Cardassians is used superbly for drama, action and character moments.
Adding Worf to the mix of DS9 was a fantastic decision, as a strong character is necessary for many of the subsequent stories and Michael Dorn's presence improves the entertainment value of the cast. He has the right mix of heroism and deadpan humour.
Most characters have enjoyable moments such as Dukat, Gowron, Quark, Garak, Odo, Sisko, and Worf. Classic scenes for me include Garak's measurements, Quark's root beer, and everything involving Worf, particularly his introduction.
There are some scenes that feel like their purpose is to help stretch the running time and slow down the momentum slightly, but they are certainly not badly made.
Visually it's excellent, especially the action scenes, which are some of the best from this era of Star Trek.