ABC

Over a number of episodes, Owen (Kevin McKidd) had become attached to Ethan, a kid whose parents were both in dire straits (but now the dad is recovering). When it seemed like Ethan might be orphaned, Owen suggested that he might adopt Ethan, which convinced Cristina for once and for all that Owen needs to have a child. Which she does not want. They're doomed, and it's sad. (By the way, Owen sending Ethan and his father off in a helicopter during what seems to be a tsunami looked like it would be a disaster, but they get off OK, apparently.)