This episode was really special, I can't believe no one has reviewed it yet!
The flashback was the starting point to the story of one John Casey. In this episode we got to see one of the many things that make John the kind of guy that he is: a tough full-of-quips guy, but like everybody else, has a past that helps explain his attitude. In the flashback we get to see how he thought he lost his love, Ilsa, supposedly a photo journalist, to an explosion in Chechnya. Then we return to the present where Chuck finds her name in a bad guy reunion to be held in the Grand Saville hotel. Chuck tells John about that and the three along with Sarah are sent to investigate the reunion.
As the episode goes on, we get to see a really nice connection between Chuck and John, the guy that has to get rid of Bartowski when the new Intersect is ready. Chuck, though, manages to get into the heart of John by getting the whole relationship with Ilsa out of his system, which translates into a development in Casey's situation, and a demonstration that he's also a human being.
At the end of the episode, John has to let Ilsa go, but now he knows that she's alive, and he's glad about that, because he knows that love is there for him, as it is for everybody. I think this is a very nice episode, and a very good build-up for the following episode, Chuck Versus The Marlin.