It's all about relationships.
Derek and Addison are going to a marriage counselor. They have an argument about whether Addison should move to Seattle or not, which is what Derek wants, and Addison doesn't want to move, but what she does want is for Derek to not talk to Meredith anymore.
Cristina and Burke have a disagreement. Burke wants to tell the Chief about their relationship before he finds out himself, but Cristina doesn't want to risk her career, or her reputation.
//If you don't want to read a spoiler, skip this paragraph.
Izzie isn't happy about her date with Alex, and she still isn't speaking to him. At the end of the episode, he does something that finally makes things right between them.
//End of spoiler.
This episode was all about relationships, really. Get your tissues ready, because if you're emotional like me, you're going to cry.