- You may be looking for the televised Sally Sparrow.
Sally Sparrow was a twelve-year-old student with whom the Ninth Doctor tried to communicate from twenty years in the past.
Sally was born in 1993. When Sally was young, she used to stare at the flowered wallpaper in her room in her aunt's cottage in Devon. She liked to pick at it as if they were real flowers. As "nostalgia", every time Sally came to her aunt's, she touched the flower to remember.
On 22 December 2005, Sally visited her aunt again while her parents were away. In her room, she mindlessly tore at the piece of wallpaper she picked a piece off when she was three. She kept tearing, and found a message reading, "Help me, Sally Sparrow!" She tore off another piece of wallpaper, and found a date alongside the message: 24/12/85. Sally asked her aunt about that day, but her aunt didn't remember, other than that she always held Christmas parties at that time and there were photographs taken.
That evening, Sally's aunt put a box containing photographs from 1985 on Sally's bedside table. In one of the pictures near the bottom, there was a picture of people dancing, along with the Ninth Doctor looking directly at the camera holding a sign reading: "Help me Sally Sparrow!" Another image had the Doctor holding another sign, reading: "Look under the wallpaper again." Under the wallpaper was another message, explaining that to prove this wasn't a trick, Sally had to think of a number and look at the furthest tree in the garden. Sally thought of the number, and found a tree with the number carved into it.
Sally later peeled off another piece of wallpaper, and found another message from the Doctor, reading: "Top shelf in the living room, right at the back." Sally found a video tape with her name on it, and put it on.
The Doctor appeared in the video, asking Sally for questions, and pointing out he could hear her. The Doctor answered Sally's questions, explaining he was a time traveller, and had parked his TARDIS in Sally's aunt's shed when it "burped" and jumped forward in time twenty years. The Doctor asked Sally to bring it back. He clarified that he couldn't really hear her, but just knew what was going to happen in the conversation, because he had a finished copy of Sally's homework, which spoke of this conversation.
He explained that "some sort of spy" in the future had saved his life in Istanbul and handed him the essay, knowing that he would need it some day. The video then showed a version of Sally from two hours in the future, landing the TARDIS in the room. She explained that the reset button next to the TARDIS phone could home in on the Doctor's watch, and that she knew this because she told herself.
The Doctor said to Sally that before she started any of this, she should do her homework. The Doctor also explained that the spy from the future was Sally. After spending two hours on her homework, Sally hit the reset button and took the TARDIS to the Doctor in 1985. She gave her past self instructions on how to do this on the Doctor's video and returned to 2005.
Many years later, Sally became the spy and saved the Doctor's life from the second of two Sontarans after the Doctor was in a sword fight in Istanbul. She gave the Doctor her homework and, knowing it would be a useful means of communicating with her twelve-year-old self, told him to keep it with him no matter what. (PROSE: What I Did on My Christmas Holidays by Sally Sparrow)