It was a busy day in the hospital on New Earth in the year five billion and twenty, when a strange creature was brought through the doors of the hospital and laid upon a bed - a young-looking human man in odd clothing, clearly far out of his time. The Sisters barely had time to puzzle over why he was dead, when he bore no wound or sign of poison, before he was awake, coming to with a gasp, and then glaring at the cats under the lights.
"Where am I?" he paused. "When am I?" Then, sitting up, "I want to see the Face of Boe."
"Who are you and why should we signal him? He travels far and wide among the worlds and may not care to come for a stranger."
The young man shook his head. "He'll come for me. Tell him Captain Jack Harkness wants to see him."
The Sisters turned to do his bidding, but before they could move more than a few steps, a shimmering light appeared and the Face of Boe arrived in the room.
"Leave us," the Face said, deeply calm. Once the Face and the Captain were alone, the Captain wasted no time in rushing to the tank that held the Face, hands pressing against the glass.
"I have not forgotten you," the Face said, still calm and quiet.
"Have you forgotten Ianto Jones?" Jack demanded. "Have you forgotten Steven Carter?"
"Ianto...Jones," the Face murmured. "Our beloved. Never." A pause. "Steven...Carter...and our daughter...Alice. Never."
"Then how do you go on?" Jack asked, tears coming into his eyes. "How do I survive, when all I do is lose the ones I love? How do I go on surviving, millions, billions of years, until I'm...you, and I'm calm and I can say their names without grief?"
"Everything has its time..." the Face whispered. "And everything dies...except what matters most of all."
"And what is that?" Jack said bitterly.
Jack knelt on the cold floor, hands pressed to the glass of the tank, eyes meeting those of his older self. "That's cold comfort to a man who's just lost everything he ever cared for."
The Face regarded him with compassionate eyes. "It takes us...many years to understand this. We have outlived...all our children...all our lovers. We have walked...the slow path...until our feet burned...and our body decayed. And at last we understood."
Jack turned away, a hand across his eyes. "What did you understand?"
"This was never about...me. This was...a gift."
"Seems more like a curse."
"It was an...act of love...and life...from a woman...who loved us. You must go on, Jack."
"How?" Jack looked at the Face once more, tears streaming from his eyes.
"With hope and with love...you saved the world...you will do so again...and again. You will lose and your heart will break...countless times. You will...find your way...one step at a time...until you are here again...and all you feel is love and memory...until time has faded all...the grief...and left only...the sweetness."
Jack slid to the floor next to the tank, hands covering his face, weeping. There was silence for a long time.
"My beloved...younger self," the Face whispered at last. "Go home."
And stars swirled around both of them, enshrouded in the smoke of Boe. It was hours before the hospital staff dared to peer in the door, only to find the room empty.
Jack awoke on a green hill in the sunshine. Below the rush and shine of Cardiff glittered. He stood up, eyes searching the horizon, and after a long pause, slowly began the long walk down the hill into the city.