7. Just Save Someone (The Fires Of Pompeii)
This leads almost directly on from the Runaway Bride, despite there being a season's gap between them. This doubles down on the need for someone to stop The Doctor from time to time - and that person's name is Donna Noble.
The Fires of Pompeii is one of the first episodes in the fourth season and as such, The Doctor and Donna were still getting used to each other. She rejects out of hand the notion that there are fixed points in history and sets about saving the people of Pompeii, only to find that their overwhelming numbers prove impossible to save. She turns to The Doctor and he begins to travel away, consumed by grief at yet another historical catastrophe.
And then - tears streaming down her face - she manages to get through his shield and speak directly to his hearts. Save someone, she says. Just save someone. He takes the TARDIS back, and this decision has lasting repercussions for The Doctor.
As he would later reflect, he would learn why he frowned that face.
Long after Donna left the TARDIS, this memory saves The Doctor again.