Spoilers: Chapter one.
River started to feel something about Donna after she kept asking her why wasn't she with the doctor in the future. Donna looked at River in a certain way that made her feel as If her heart would pop out of how hard it was beating.
After River and Donna met, they both felt something, a kind of click between them. Something they weren't very sure of talking about yet. They didn't have their feelings very clear but the looks they made between each other, betrayed them. The doctor hadn't noticed anything yet. But Donna had to talk to River. She wanted to know what was happening.
They were still in the library figuring out how to make the dark creatures go away. There was a moment when Donna and River were next to each other waiting for the doctor to give them a signal to move on with the plan. Donna felt the necessity to talk to River.
Donna: River, I need to know something. Why do you know my name but you don't know why I'm not in the future with the doctor.
River: Donna, it's hard to explain.
Donna: Well try then, please. I need to know.
River: I know. I know how you feel. It breaks me to know that the doctor doesn't remember me when I know everything about him.
Donna: So, you can understand me. Please tell me.
River: The truth is I don't remember. I can't remember.
Donna: What do you mean you don't remember? You've got that book, haven't you? Spoilers book. Look in there, for me.
River: You're not in there.
Donna: What do you mean I'm not in there? So if I'm not in there, it means I'm not with the doctor, so where the hell am I? Am I dead?
River: Donna please...
Donna: Tell me... am I dead?
River: I...
In that moment Donna ran away to another part of the library, not far from where the rest were. But the doctor noticed her and started to scream at her desperately, trying to make her come back.
Doctor: Donna! Come back! It's dangerous!
River: I'll get her.
Doctor: Please… take care of her and bring her back.
River: Don't worry, I will.
River ran after Donna and found her where they were when Anita died. River saw her crying and hugged her.
River: I'm so sorry. I didn't mean to make you feel this way.
Donna: (wiping her tears) You didn't answer my question. Am I dead in the future?
River: No, you're not dead. But that's all I can tell you. I can't tell you anything else because I'm not sure.
Donna: Well, at least I'm not dead. It's all I needed to know. (Small pause) All my life, I felt like nothing. I felt, completely useless. When I met the doctor, he made me feel like I was actually someone. I feel like a totally different person. That's why I want to be with him, forever. This is where I have to be. This is my destiny.
River: Donna, whatever happens, please remember, you are "the most important woman in all creation.
Donna: (looking at her surprised) That's what Rose said to me.
River: See? It's not just me. All of us think of you as a wonderful companion. The doctor chose well. He needs you and you need him.
Donna hugs River while tears come down her cheeks. They both felt so happy in each other's arms. Donna felt protected and safe in Rivers arms. And River felt she had to protect Donna, make sure nothing happened to her, as she promised the doctor she would.
River: Donna, we must go back. It's not safe for you to be here.
Donna: all right.
River: Don't worry; I'll make sure nothing happens to you. I'll take care of you.
Donna smiled at River as a sign of agreeing with her. River smiled back and took Donna's hand and they go back to where the doctor was.