This is one of those episodes that tugs at your heartstrings. While showing Booth's boss some facial reconstructions, Brennan questions him about his daughter's cancer (after several stern warnings from Booth NOT to). When she begins investigating, she discovers that the bone graft the girl had gotten was from an old man with cancer.
Booth and Bones and the rest of the gang put all their effort in trying to solve the case of where the illegal transplantation took place. B & B work well together on this case, and all get emotional. Angela is frustrated that she can't do anything to help the girl, with whom she's become attached.
One thing I really like about this show is how everyone just comes to care for each other. Even Booth has seemed to accept Brennan's co-workers and accept them as part of her life. They all have big hearts and want to do what's best to make the world a better place.