In her second adventure, we get the first mention of what will go on to be this Doctor's defining arc.
While hunting for her missing TARDIS, the Doctor and her friends meet a pair of aliens competing in a race to reach what they call "the Ghost Monument" — a structure that fazes in and out of existence, found on a planet called Desolation. The Doctor joins this race, because the so-called "Ghost Monument" turns out to be her TARDIS.
During their journey, they encounter a species called the Remnants. Long story short, the Remnants are basically sheets capable of hunting and killing people; also, they can see into the Doctor's mind. The linen-like monsters accuse her of fearing her own newness, while also claiming they see more of her past than even she remembers.
"We see deeper, though, further back," they whisper. "The Timeless Child."
These words strike a nerve, and the Doctor asks them to elaborate. They say, "We see what's hidden, even from yourself — the outcast, abandoned and unknown." She has no idea what they're talking about, and circumstances force her to escape before she can find out. This the first time we're hearing about this so-called "Timeless Child," and it's a little incredible how close this apparently vague and cryptic dialogue comes to explaining the Timeless Child's whole deal.
