SPOILERS: Excerpt from the end of "Winter," final episode of the Big Finish Doctor Who story Circular Time, showing what's happening inside the fifth Doctor's mind during his regeneration scene.
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The Master had used Kamelion to create a dream-world, the hallucinatory equivalent of Castrovalva, trapping the fifth Doctor and preventing him from reaching out to his old companions telepathically to help him hold on. (He was dying for hours, using sheer willpower to stave off regeneration as long as he could).
In Circular Time, Nyssa, who is slightly telepathic herself, hears the Doctor's cry for help and uses a dream-therapy device created by her husband to reach him and guide him through the hallucinatory blizzard in which he's lost.
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Every time I listen to this, my eyes water. But it makes me feel a little better about Caves of Androzani.