You discover a letter you wrote to yourself ten years ago. What does the letter say? Write the letter.
"Now where did I put that thermostatic coupling?" The Doctor rummaged through the boot cupboard, tossing aside various and sundry items of footwear without a second glance. "I could have sworn I put it . . . Hullo! What's this?"
He plucked a small white square of paper from its place in the toe of a Roman Centurion's sandal. Unfolding it, he saw the following lettered in a bold hand,
"Hullo Me.
If you're reading this, you're rummaging through the boot cupboard in search of the thermostatic coupling component.
Well, then. I've a bit of advice for you. You'll ignore it, of course, but there we are. I have to at least try.
1. Do not trust the High Council. Yes, I know. It rather goes without saying, doesn't it? But this time it's especially important.
2. Spend more time with Peri. You'll thank us later.
3. Never lose hope. Not everything is as it seems.
4. Don’t bother seeing the DaVinci Code. The book is better. Leonardo agrees.
5. Nemesis is on the way back. Try not to forget the date this time.
~ The Doctor
p.s. - The component isn't there. It's in the wardrobe, black frock coat, inside left pocket, exactly where we left it.