Liminal Beings.
This story occurs probably after the televised story Black Orchid, and certainly before Earthshock...
Preface
The marsh creature recoiled, but for some reason it didn't seem quite so violently affected as the others they had come across. More likely, Adric reasoned, it was one of those Marshmen. The Doctor had explained about their adaptability. The thought shocked him. Could they already be developing an immunity?
Adric's cylinder spluttered and fell silent. He tried the valve again, to no avail. Checking the pressure gauge, he saw it read zero. "Varsh – my cylinder's run out!"
"Then leave this to me – get the others back to the Doctor!"
"I can't leave you on your own."
"Adric – don't argue, go!"
...
He was moving along a corridor, through hoops of polished steel, the smooth and shiny floor slipping below scuffed with marks where running feet had been. He was neither running nor walking. He seemed to have no legs or feet at all, but instead appeared to be effortlessly gliding, sailing through the air like a boat moving on the water, and gazing with interest at the metal walls as they drifted slowly by.
He was starting to enjoy himself. He was flying! But he felt that, if he really wanted to make the most of the situation, he would have to learn to control his flight by reaching out perhaps and making contact with a surface. If he could just catch one of the girders with a fingertip maybe...
But it was no good: the walls were beyond his grasp and he couldn't find his legs to kick against the floor, so he just carried on floating without any means of control.
The corridor up ahead forked and briefly he wandered how he would be able to fly into the right fork or the left, and which one it should be, and as he drifted into the left fork with no effort on his part whatsoever he began to feel a sense of unease at his situation, that grew into a fearful anticipation of what lay further on.
The corridor here was brighter. There was a light at its end: a broad stark block of light and a movement, a shadowy flux within the light and suddenly Adric felt a panic of anxiety grip him, and an urge take hold to reach the light that was changing shape now, shrinking downwards as if being squeezed from above, a darkness descending upon it and crushing it flat. He reached out his hand in his panic, stretching his fingers towards the light that was a thinning silver bar now and woke up.
He was in his bedroom. It was dark. His bedroom door was shut and there was a sliver of light coming under it from the corridor outside. He could hear his clock on the table nearby and further away, the hum of the engine, constant like a heartbeat.