Where's Wally?

Where's Wally? was a series of children's books from Earth that challenged the reader to find the eponymous character within its illustrations.

The Eleventh Doctor once spent several days trying to find Wally (PROSE: The Day of the Doctor) in Advanced Quantum Mechanics, (TV: The Day of the Doctor) a book of complex temporal theory. He began to suspect that Wally couldn't be found in every book, but concluded that he couldn't be sure. (PROSE: The Day of the Doctor)

Whilst in Rupert Pink's bedroom in the 1990s, the Twelfth Doctor was frustrated that he couldn't find Wally in the book that he was reading. Rupert then pointed out that the book wasn't part of the Where's Wally? series. This annoyed the Doctor, who had presumed that it was possible to find Wally in literally every book. The Doctor grouched he would be needing a few years of his life back. (TV: Listen)

Behind the scenes
The Doctor Who book Where's the Doctor? followed the same style as the Where's Wally series. It is also a regular feature in Doctor Who Adventures magazine and recent editions of the Doctor Who Annual. Where's Wally?