Disclaimer: I still don't own Doctor Who.
The TARDIS clock said it was the middle of the night. The Doctor had woken up after a terrible nightmare in which River Song died as he watched. When he had woken, he found that reality was not much better. River would die in the morning, and it would be his fault.
For now, though, he could just lie there and pretend that that would never happen. He could convince himself that he will save her. He felt her bare skin warm against his and planted a kiss in her mass of curls. She seemed to have felt this, as she subconsciously pulled him closer to her. He did the same, pretending that maybe if he held her tight enough, she wouldn't have to leave, wouldn't have to die, and they could just be like this forever.
River woke to find the Doctor crying, silent tears rolling down his face. She again wondered if she had done something to upset him. "What's the matter, my love?" she asked in a pained voice.
"Nothing, nothing," he answered quickly as he attempted to wipe the tears off his face.
Of course, she knew when he said "nothing" something was always wrong. She didn't say anything, only buried her face into his chest and went back to sleep. The Doctor slept too, hoping that maybe he would wake up to find out it was all just a cruel nightmare.
The Doctor woke up hours later and reached out for River. She was not there. He was immediately wide awake. He began frantically searching the empty bed sheets as if she were just hiding. He hurriedly pulled some clothes on and began a more in-depth search of the TARDIS. He looked in all of River's favorite rooms, all to no avail. Soon he became practically hopeless and fell to a heap on the floor of the console room.
The Doctor was too panicked to cry. His mind was going a million miles an hour. There was so much he hadn't said, so much they hadn't done. All of these hopeless, desperate thoughts flooded his mind, and he began begging the TARDIS to send River to him on the off chance she was still there. He was about to completely abandon any hope he still had when he heard a worried voice behind him.
"Doctor? Doctor, what's wrong?" River Song rushed to his side, wondering what he could possibly be so upset about. She sat down next to him on the floor. "River!" he practically shouted. "River, where were you?"
She looked confused. "I had only gone to get some breakfast, sweetie." The Doctor's expression was desperate now, at least more so than before.
"You can't leave! Promise me you won't! There's no way I could possibly fly off to see the universe without you! Just stay a little longer." Though the tears had not yet fallen, River could hear them in his voice.
"Sweetie, I've got important things to do today. You'll need to take me back to Stormcage eventually."
"Not today. Please, River just one more adventure." She did not fully grasp why he was acting so strangely, but she accepted.
The Doctor's first reaction was sheer excitement. River wouldn't die today. He didn't put in any coordinates. "Just send us where we need to go!" he yelled to his beloved blue box, and she sent them whirring through the time vortex.
River, by this point, was now thoroughly confused. She had let it go when he had cried at the Singing Towers, or when he brushed of her questions in the night, but now she could not take it anymore. Something was not right. Something was upsetting the Doctor, and she was determined to find out what. "Doctor, you know that I would follow you anywhere in the universe, —as I'm sure that's exactly what's happening now—but I need some answers. What's going on? Please, tell me. I know you're used to doing things alone, but I'm here now, and I can help if you only tell me."
The Doctor actually seemed to think this over for a moment before answering. "I'm sorry, River I just can't. So can we at least enjoy this last adventure?"
The Doctor's face paled, and he became stiff as he realized what he had just said. He immediately began searching for a cover-up. "Um, er, yes. Last. As in this will be our last adventure together before I must return you to prison."
River's expression was suspicious at first, but then she thought that she probably shouldn't press the matter any further. Spoilers.
The rest of the journey was fairly silent. The Doctor was pressing buttons that didn't need pressing, while River quietly undid the mistakes. After only a couple minutes the TARDIS landed, and the Doctor jumped down the stairs to open the door for River, both of them trying to look excited for the other's sake.
River smiled at him with her usual mischievous grin before actually looking at what lay outside. When she did, though, she became confused again, and turned to the Doctor, saying, "I wonder why she brought us here again. We just visited the Singing Towers."
The Doctor's heart dropped as he grasped the situation. He realized that this was the TARDIS's way of telling him what he needed to do and what had to happen. He slammed the door quickly before River could exit and began to shout. "What are you trying to tell me? This was a cruel joke, even for you! You know exactly what I mean!"
River couldn't stand him yelling at the TARDIS like that, so she stepped forward. "Sweetie, you and I both know that something is terribly wrong, and the sooner you tell me, the sooner I can help you," she said pleadingly.
The Doctor looked torn. As if he really wanted to tell her but couldn't for some reason. He knew that was exactly the case. However, instead of letting her see through him, he put on his best brave face and began babbling as usual. "So since the TARDIS is being a bit unruly today, I suppose I'll just punch in the coordinates and take us somewhere amazing. How 'bout a planet where it is always sunset? The day never changes, so it never becomes tomorrow!"
River sighed and decided that it would just be better to go along with whatever he said. "Sure, dear. It sounds magnificent."
The Doctor pressed a few buttons and pulled a few levers, and soon they landed. This time the Doctor peered out the door to make sure that they had reached the correct destination. He saw where they were and sighed. "Stormcage." This was it. He had been defeated by his own TARDIS. He knew this was what must happen, but he still found it impossible to accept. "River," he said softly, "I'm afraid this will have to be goodbye."
River could see exactly how much this was hurting him, and decided not to speak. She instead wrapped her arms around him and brought his lips to hers. He gladly returned the kiss, relishing the last few moments they had left. She began to kiss him more forcefully, and the Doctor pulled her closer, not wanting to miss one thing. He was about to lift her off her feet when she pulled away and said, "I really must be going now."
The Doctor did not cry. He walked her over to the door and opened it, their hands still firmly clasped. He looked her in the eyes, and she could see every single year reflected back at her. "Goodbye," he whispered as she walked out the door. He held on to her hand for as long as he could, and then . . . he let go.