This is one of the most emotional episodes of the whole series. From when scene when Valenti comes in and Maria is the first to find out, to the last moment this episode is pretty intense. When Liz goes to Alex's car, part of me really believed that she was crazy as she would be called later.
Jason Dohring was also great in this episode (I loved him in Veronica Mars) and when Liz rips into him on the field it is really classic. The saddest part comes from Isabel whose dreams of Alex are so bitter sweet and your heart breaks for her when she had just gotten on track with Alex and then he dies. She is heartbreaking in this episode. The funeral scene is so sad and Maria doing Amazing Grace was really touching.
Then in Alex's bedroom when they divide between humans and aliens it is so reminiscent of the first season, that you know whatever comes next is going to be great.