This is a quick response to all those "Susan is able to live with Caspian for the rest of her life and they are very happy, so who cares about the consequences and the fact that if they die in Narnia they'll die back home and what would happen to their parents and everyone who cares about them then" fics.
Spoilers for the movie Prince Caspian and the books Voyage of the Dawn Treader, The Silver Chair, and The Last Battle.
Susan and Peter stared at the great Lion in shock.
"Never?" Peter gasped. "But—Aslan, Narnia is our home more than England could ever be. How can you keep us from it? What have we done wrong?"
"Wrong?" Aslan repeated. "You have done nothing wrong. Narnia has simply taught you everything it can. You have lives to lead in your world. Your time here is over. It is Caspian's time now. It is time to move on."
"Aslan," Susan whispered, "I think I'm falling in love with Caspian. Isn't there—isn't there any way I can stay and—and—" Her voice broke as she choked back a sob. "Oh, Aslan, I can't leave. Not now."
Aslan paused in his steps and turned to gaze directly into the Gentle Queen's eyes. He watched her for a long moment, and then nodded. "If that is truly how you feel than you may remain."
"Oh, Aslan," Susan gasped, "thank you. Thank you so—" She stopped at the serious look in the Lion's eye.
"I am not finished," He rumbled. "You may stay, but your siblings may not. And this time, you will not return years later to your world as though no time has passed.
"Nothing ever happens the same way twice, Queen Susan. If you stay now, then you will live here and die here. Your siblings will return with nothing but a corpse. Your parents will not truly understand what happened to you, and neither will your friends in your world.
"You may have Narnia and the man who lives here, or you may have your world and those who live there, but you must choose one."
"Choose?" Susan whispered. "How can I choose between family and love? How can I choose between my siblings and Caspian?"
Aslan did not answer.
"Su," Peter whispered, "whatever you choose, we'll support you."
Susan nodded, and the three walked in silence for a long time.
Susan stood next to her siblings as King Caspian X spoke to his people. Though she appeared calm and collected as a Queen of Old should, a silent battle was being waged in her mind.
You cannot leave him. Not now. You know he loves you. You know what you can have.
What about what you already have? What about Peter? What about Edmund? What about Lucy? It's not such an easy thing to leave your siblings.
They'll understand. Surly they will.
They'll try. That doesn't mean they won't feel abandoned and betrayed.
Susan watched as Peter announced that it was time to leave. She heard Caspian promise to protect the sword, naïvely certain that the Pevensie siblings would return. She heard Aslan's voice in her head, gentle but firm.
It is time to decide, child. You cannot delay it any longer.
Susan bit her lip. She gazed beyond Caspian to the Telmarians who were watching Aslan and the Pevensies with thinly-disguised distrust and, in some extreme cases, hatred. In an instant, her decision was made.
"That's just it," she told Caspian, "we're not going to come back."
The next thing she told him hurt, but it was true. It couldn't have worked between them. Not because of her, not because of him, but because of Narnia.
Her Narnia, the Narnia that she had formed and ruled alongside her siblings, had not existed for 1300 years.
His Narnia, the Narnia that she, Edmund, Lucy, and Peter had helped him claim and the Narnia that she would help him rule if she married him, was darker, but also more grey. Caspian would have to deal with corrupt lords as well as loyal Animals and other creatures. While in Narnia's Golden Age it had been easy to tell who was a friend and who was an enemy, Caspian would always carry the knowledge that it could be dangerous to trust anyone too much until he was sure of their motivation.
Susan's Narnia was a Narnia of beauty and glory, and she simply did not belong in the Narnia of assassinations and political intrigue.
She couldn't resist kissing him one last time before she left but, as the clear air and bright courtyard dissolved into the dank, musty subway station, she told herself that she had made the right choice, the only choice.
She wasn't sure if she was lying to herself or not.
Months later, when Lucy and Edmund returned from the trip to Narnia that she had not been a part of and told her that Caspian had fallen in love with someone else, she told herself that she must have made that right choice. After all, he had moved on, and this star's daughter would make a better Queen of the modern Narnia then she would.
It was harder to believe than the last time.
It was when Eustace returned from Narnia the second time and told her that Caspian had died of old age that she began to despair.
When she had finally cried herself out, she made the decision to stop believing in Narnia, Caspian, and Aslan. After all, if she had never had them, then she could never lose them, and if she never lost them, how could it hurt?
This time, she didn't even try to pretend that she had made the right choice.
Years later, after she tried and failed to forget Narnia, and lost her siblings in the process; after a single train accident robbed her of all of her family and many of her friends; after she admitted to herself that Narnia and everything that happened there had been real, and there was no use denying it; and after she found the man who woke up her heart once more, the only person in England who brought out the part of her that was still Queen Susan the Gentle of Narnia, she realized something that a part of her had know for years.
Aslan had known exactly what he was doing when he offered to let her stay in Narnia. He had given her two choices, and she made the right one.
So, yeah. There you go. Please review and tell me what you think.
Romance is not my strong suit, so anything you have to say is greatly appreciated.