The one where Victoria learns the Jewel of the Lotus prayer...
I keep harping on about it, but Wolfe Morris really is the best thing about this story. The way he spits "Song-sten" gives me the willies, and the scene where Padmasambhava laughs maniacally at the death of Khrisong is truly creepy. His laugh sounds like a croak, a creaking gate, but is filled with malevolence. Wolfe Morris deserves much more praise for his performance in this story than he ever gets, especially as his connection to the Yeti myth goes back years. He played the character of Nima Kusang in the BBC Sunday Night Theatre play The Creature in 1954, and reprised the role when Nigel Kneale's script was remade by Hammer for the big screen as 1957's The Abominable Snowman (starring movie Dr Who Peter Cushing). His casting in Doctor Who was no coincidence.
Morris gets over Padmasambhava's internal conflict so well. His voice wavers and falters as the Great Intelligence tries to subsume his right-thinking self (the bit where he says: "You will no-o-o-o-o-o-w..." and just tails off as he loses control is heartbreaking and chilling).