Season three fleshes out Meredith's relationship with her Alzheimer's-ridden mother, the great surgeon Ellis Grey. It stands out as one of the series' darkest seasons as it explores Meredith's struggle with depression.
In this episode, Derek pulls Meredith out of the water. His rescue is more heartbreaking than heroic. For most of this episode and the next one, Meredith looks dead (although we all knew that she was going to make it out alive, because duh, she's the main character).