Mutternot's Tales is a Blackout Challenge on Floor 6 introduced in Series 4.
The Caretaker's Explanation to the Game:
"A place for everything and everything in its place. Ms Mutternot is not happy. The Tower Library is completely con-fuddled. Unfortunates must carefully put the books back in their rightful place. But, the Tower is prone to power cuts and someone is trying to upset the librarian. One of them cannot be trusted."