The Donna-human looked funny in the Doctor's clothes.
Ever since he had refused her shopping trip idea in future Earth, claiming it would cause a rift in time and lock her into an alternate universe if she herself started fashions meant to be worn only later, Donna had one thing in mind: disobeying him and his logical reasoning for that shopping trip.
People expect women to cry, faint, or beg. Not Donna. She'd take matters into her own hands, whether she needed to or not.
The TARDIS saw that in the Donna-human's mind. She liked this pink and orange human. That was why the TARDIS decided to let the Donna-human play her game.
Donna was wearing the Doctor's blue suit, since he was wearing his brown one, as she walked into the console room. She had no idea how to fly the TARDIS, so she assumed her best bet was to impersonate the irritating Time Lord that was in the library tinkering, making some kind of machine to cure all ailments or something equally irrelevant.
She was going to ask the TARDIS to take her shopping.
"Weeelll," Donna drawled. "Since the coordinates of the Egg Beater Constellation number one are overshadowed by the proclamation, and the flux of, er, suspension is unstabilized, TARDIS, will you take me to a 22nd century shopping mall?"
There was silence, then the unmistakable sound of the engines whirring to life. She punched the air, then remembering she was impersonating the Doctor, lowered her voice an octave and exclaimed: "Allons-y!"
Donna was quite confident the TARDIS couldn't see.
The TARDIS was quite confident the Donna-human wasn't as smart as she'd thought at first.
Donna opened the doors, waiting to see the 22nd century in all its shopping glory, but she only saw a lone figure surrounded by books. She blinked, and saw it was the Doctor carefully cutting wires on some kind of contraption. He looked up, and saw her carrying empty canvas bags and a credit card.
"What're you doing?" he asked her, looking back down at his lump of wires.
"Just looking for the… the spa room," Donna invented.
"Down the hall, to the left, another left, the hall with vines growing on it, right, left, right, then down the slide," he said, not looking up.
"Thanks," she said, stepping backwards, unsure of how he would react once he realized she was trying to fly the TARDIS on her own.
"Wait!" he said. Donna's breath caught. "Why do you need those bags?" he asked suspiciously.
"To carry my bathrobe," she said, glad for that last bit of cleverness. The Doctor nodded and once again looked down. Donna let out a breath she didn't know she'd been holding, and tried to edge away.
"Wait!" he said again. "Why are you wearing my clothes?"
Damn.