Solty Rei is getting darker as the series progresses. People die a lot. A lot of time is spent showing the remaining characters in mourning. Since most of the people alive in this world are in mourning for loved ones lost twelve years earlier in the Blast Fall, there is a lot of grief around.
A couple of episodes were spent just mourning Rose. Poor Solty, unable to please Ray, left the city and nearly burnt her circuits out crossing a desert. She was repaired by an old man, Joseph, and befriended a young boy, Will, whose ambition was to fly an aeroplane he had built from scrap. He, too, died of an illness he had incurred as a result of the Blast Fall twelve years earlier. Another loss for Solty.
Once Solty was back in the city, the series turned darker. Rose has been brought back to life as part of the RUC (whatever RUC is - some sort of ruling body with military security powers). It is not certain yet how much of her past life she remembers. Mentioning Rose's return to life is not much of spoiler since she was obviously one of the main characters and it was really expected that she would come back to life. However, her success in the program that revived her has been bad for the existing RUC girls.
Besides being darker, the series has also become denser. Look away now, and some important plot point might be missed. It turns out that the Director, who has a 'Phantom of the Opera' style half mask, is in fact quite murderously mad. His aim is simply to get off the planet.
The race is on to see if enough people can survive for long enough to thwart the Director's murderous plans.