The one where the seaweed creature is defeated by Victoria's screams...
Ah, the old new theme tune is back! That feels better. This revamped theme feels much more energetic, which suits the Troughton era better I think (just as I much prefer the galloping version of the theme tune in 2008's Series 4 to the earlier Eccleston version).
It's a blissful irony that what finally defeats the weed creature is Victoria's ear-piercing screams. The Doctor notices that the possessed Robson shrinks away when he hears Victoria's scream, and realises that sonic power is the key to defeating this underwater menace. After 11 months of listening to the power of Deborah Watling's formidable lung capacity, I can well believe that it would destroy just about any alien menace! It's actually a really poetic way to bring the adventure to an end, especially as it is Watling's swansong.