Donna decided that they were going celebrate Christmas despite the Doctor's indifference to Christmas which he voiced by saying. "Christmas...meh."
She asked the TARDIS if there were any decorations or a tree on board.
A burble told her that there was and a door appeared that led to a set of wooden stairs...the attic she guessed.
She entered the attic and marvelled at the things that were inside. There were wooden chest that had letters of the alphabet on them starting at A and ending in Z. There were boxes with the Doctor's own language written in what looked liked gold.
She knew not to touch those, they were probably private and she would never pry.
She moved further into the attic and stopped when she found what she wanted...a collection of boxes with tinsel and baubles and a tree that she was sure couldn't be real, even though it smelt and looked real.
She went to pick up one of the strands of tinsel when she saw it, peeking out from the branches of the tree.
She yelped in surprise and the one that had surprised her squeaked its own surprise.
Mice...she thought...there's mice in the attic of a spaceship.
"I'm not having that," she said and as she moved, there was more rustling...this time in the box of decorations.
She yelped again when three more mice appeared amongst the tinsel and looked at her like she was the interloper.
"Doctor!" she yelled.
The sound of running feet told her the Doctor was coming.
He appeared at her side, sonic in hand, prepared for danger.
"What is it?"
When no slavering monster appeared he frowned and then said. "What are you doing in the attic?"
Donna blinked. "Mice, you've got mice!"
The Doctor didn't seem surprised. "Oh, you've met Maurice's children then."
Donna gave him an indignant look. "Maurice?"
The Doctor looked in the box and made a squeaking noise and four little faces appeared out of the tinsel.
"He got on board last Christmas...I didn't have the heart to turf him out, what with the snow and the cats."
Donna tensed as the mice crawled onto the Doctor's hand. "Well I say he...turned out he was a she but the name sought of stuck. Anyway, she was pregnant and gave birth inside the TARDIS thermal-couplings."
"And you let them stay...but mice."
Donna looked at her sadly. "Mice don't live very long Donna, not even in the TARDIS. Mice get pregnant very young and Maurice died just after giving birth...so I adopted them."
He stroked one of the mice gently with a finger and Donnas' face softened...it was just like the Doctor to never let anyone or anything be alone.
She put out a hand. "Can I?"
The Doctor smiled and let the mice scurry onto Donnas' hand.
"The one on the left is Wally, that's Trixie, that's Barry and the one about to climb up your sleeve is Maurice the Second."
Donna smiled but then saddened. "But they won't live very long?"
The Doctor grinned then. "Oh no, not these. The TARDIS is a sentimental old thing. When she realised Maurice had died she put a little of herself in the babies...these little mice are going to live until my last regeneration and then like myself and the TARDIS, they'll just fade away."
He looked a little sad then, but Donna put her free hand on his face. "And I'll be right there with you Spaceman."