There was a library in the Tardis.
It was filled with every single book ever written, breathed, or felt in history and the future. For this reason, normally Companions weren't allowed inside. Too much temptation for spoilers, really.
But as the Doctor strolled past one night (day? In space and outside of the flow of time, it was hard to keep track), he heard the soft, shuddering sounds of anguish. He frowned, paused outside the door. It didn't sound like Rose, and it certainly wasn't Mickey-the lad was back on Earth again.
The Doctor's eyes widened in shock as he realized it was Jack. Jack was sobbing to himself, a broken record of harsh coughs and shattering hiccoughs. The Doctor set his laundry down, sighing and running a hand over his scalp. What could Jack have found? Had he searched for the future of the human race and found it lacking? Had he read the fate of his friends and family? The Doctor pushed the library door open slowly, stepping inside as quietly as he could.
"Jack?" he called softly into the cozily lit room.
Jack was curled into himself, tucked away in the corner of the library on the floor, an open book carelessly tumbling from this lap, pages ruffled and tear splotched.
"Oh, Jack," The Doctor moved to Jack's side, sitting next to his shaking form and wrapping an arm around his shoulders, "What was it?" He didn't chide, didn't admonish that Jack should have known better, didn't say that most of the time not knowing WAS better.
Jack tried a word, but it died in his throat on the next horrible sob, and instead just plucked at the pages of his book.
Turning his eyes toward the old, old tome, the Doctor sucked in a breath.
"A History of WWII".
"Oh Jack… why?" Jack jerked away from the Doctor's touch like he'd been branded.
"I ABANDONED them!" Jack's voice cracked as he shrieked, "I abandoned them!"
"Who Jack?" The Doctor's voice was low, soothing, but in the face of Jack's pain it just sounded flat.
"All of them. I'm a coward! A fake, a fraud! I lied my way in, lied my way across the galaxy and into one of the most important wars of human history, STOLE a dead man's name-," His breath caught but before the Doctor could interrupt Jack plowed on, "I abandoned Grey, I just kept running! I abandoned John, I abandoned my Mother! I was a Time Agent, I was someone meant to protect others, I was the poster child for the entire Boeshane Peninsula and I ran away from that. For what? Two years of my memory?"
He balled his fists together and collapsed against the Doctor, tears still streaming from his face, "I've run away from everything my whole life! But do you know what's worse?"
The Doctor said nothing, just stroked Jack's shoulder, holding him.
"What's worse is that those men… they trusted me. Believed me. Followed me. And I abandoned them to die in the fucking London Blitz!"
The Doctor closed his eyes against Jack's quiet words of truth.
"I should have died with them. I should have died like the real Jack, died defending my men and those people. And instead I ran away again,"
"No! Jack, don't say th-," The Doctor started, his voice gruff in horror.
"Yes! Read it! READ IT!" Jack thrust the book into the Doctor's hands. Names of the Dead in the London Blitz. Names of the Dead… and missing.
One name was listed twice.
January 21st, 1941. Group Captain Jack Harkness. Dead by surprise enemy encounter.
Deserter. Captain Jack Harkness.
And the names of the men under the deserter's command… all dead in the bombing.
"Jack, it wasn't your fault. The lives lost in that war… no one could change it," The Doctor closed the book softly, setting it aside as he pulled Jack into a tighter hold, "It wasn't your fault,"
"You don't know that Doctor. Like you said, 'just that once, everyone lived,'" Jack stared at the Doctor, eyes wide and red rimmed, expression desperate, "I knew what was going to happen! I'm not an idiot, I knew my history even then-I had to know it because don't forget-I was a con man. I made my profit off of knowing what would happen, off of knowing my history and my major battles. I sold people things that I knew were going to have bombs dropped on them. I could have warned them, told them to get out. You saw the billboards Doctor 'Mothers, your Children aren't safe in London, send them into the Country,' I could have made a difference!"
"Jack-,"
"I was THERE! I knew what would happen and I ran away! I could have, have, stopped the atom bomb. I could have stopped the war itself. I could have-," Jack stopped talking when the Doctor grabbed him by his shoulders, shaking him lightly.
"Jack. You know as well as I do that you couldn't have done that. You KNOW that you couldn't have changed it. A paradox of that magnitude would have snowballed so completely out of control Jack…"
Jack's desperate expression clouded with pain and confusion, "But Doctor, how do you know?"
The Doctor was silent then. Weighing his answers. A shudder running through the body curled into his side spurred him on, "Jack. I know because it is the role of a Time Lord to see these… ripples and eddies in the pool of time. There are still patches where you can see clear through to the bottom, and were you to drop a pebble into them you would shatter the clear image. Change the course of the water. Where the water is rough, turbulent… you could drop a boulder into it and never change so much as a drop. The problem is that the calm pools are so often in reality terrifying and bloody moments of strife and anguish that those living in them must endure. Believe me, were you to have died with your men, it would have done little else than filled one more grave,"
Jack felt his heart constrict, "But they wouldn't have died alone,"
"No-one dies alone Jack. I promise you that," whispered the Doctor.
Jack pulled the book back into his lap and leaned harder against the Doctor, tears falling quietly now.
And the Doctor held him in silence.
There is a library in the Tardis.
It is the only room that is locked night and day.