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.

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. (TV: Listen)