Just Imagine...

Cavern of Secrets

Jack tried not to think about the fact that he'd just up and left the others without warning. He tried not to think about the team at all. He was on a planet at the end of the universe, and still reeling from the shock and joy at discovering that Rose was still alive, the reports of her death had been faked to keep her safe. That's what he should be thinking about. But his mind kept drifting back to twenty first century Cardiff, and the people he'd left behind there, as he, the Doctor (who hadn't changed a bit since they'd last met), and his new companion, Martha Jones, explored the vast network of ancient tunnels on this strange world.
Jack noticed there were tiny, glowing, blue-green crystals poking out of the walls of the tunnels, and they got bigger and showed up in larger clusters the deeper in they went, as well as getting brighter.
"Doctor," Martha said eventually, stopping to examine one of the crystals, "what are these?"
"Fantastic question," the Doctor murmured, "I don't know. I've never seen anything like them before."
"Was he always this little help?" Martha asked Jack.
"Get used to it." Jack laughed in response. The Doctor ignored them both and kept walking.
"We should be careful not to get lost down here," he said, "try and stick together, okay?"
Jack had stopped listening at that point. There was a sound coming from nearby. Laughter.
Jack knew that laugh, but he'd never thought he'd hear it in these tunnels.
"Ianto?" He asked softly.
"Jack?" Martha said, looking at him with a concerned expression, "what's wrong?"
Jack didn't answer, just ran towards the sound, brushing past the Doctor in his haste. The laughter had stopped but instinct brought Jack to it. He rounded a corner and stopped in his tracks.
Ianto turned around to look at him, face illuminated by the glowing crystals around him.
"Hi Jack." He murmured with a radiant smile, a slight trace of laughter still in his voice.
"Ianto," Jack said with a grin, "how did you get here? I-"
"JACK!" The Doctor shouted from far down the tunnel, causing Jack to look away for a second.
The tunnel in front of him shifted, and suddenly he was in the forest where he'd first met Ianto and when Jack looked back Ianto was in the t-shirt and leather jacket he'd worn that night. Ianto stepped forwards and the scene changed again to the hub's basement, Ianto with blood on his hands and tears streaking his cheeks. Then it was after Jack confronted Abaddon, in the hub when Ianto had walked up to kiss him. And then it was back to the cavern at the end of the universe, blue crystals everywhere, Ianto right in front of him. Jack inhaled sharply as Ianto cupped his face in his hands, pressing his forehead to Jack's. Then Jack smiled, leaning into the embrace, reaching up to touch Ianto's hand-
And then, with a flicker, it was over.
Ianto was gone, and Jack's feet were at the edge of a long, dark drop into the centre of the planet.
Jack stepped back, startled, then bit his lip against a sob. Of course it wasn't real. Ianto wasn't here. He was back in Cardiff with the others.
Looking around, Jack realized there were thousands of those crystals on the walls in this section of the tunnels. That must be what it was. These crystals were messing with Jack's mind. Making him think the impossible was possible. The impossible being having any sort of future with Ianto; Jack was immortal, Ianto wasn't, he'd die someday. And besides that, Ianto obviously didn't feel the same way about Jack as he did about him.
It was just a trick.
"Jack?" Martha asked, standing next to him with her hand on his shoulder, "are you okay? What happened to you back there?"
"Nothing," Jack lied, not looking at her, "I think these crystals have some kind of aura, they're muddling my thoughts."
"Wouldn't surprise me." The Doctor murmured. Then he frowned worriedly. "You sure you're alright?" He asked.
"Yeah," Jack said, once again lying as he shook himself, "just... seeing things."