The Aztecs is widely regarded as one of Barbara's best stories in Doctor Who, and Jacqueline Hill names it as one of her favourites in this interview. Here, in the opening scene of episode 3, "The Bride of Sacrifice", she saves Ian’s life in an impressive display of coolness under pressure. Annoyingly, Ian is unconscious and completely misses it.
It's a great scene, really tense and enjoyable. Barbara's on typically excellent form, and I particularly like John Ringham's expression as Tlotoxl realizes what's happening, before the camera cuts to the long shot and back again.
The video seems to be lost itself seems to be lost, but we all know what scene they're refering to!