They made the mistake of taking Rose.
The Doctor ducked behind a particularly leafy plant. He could see Rose, surrounded by her captors, just about ten feet away. They had dressed her in the traditional robes of this place, and seemed to be preparing her for some sort of ritual bath. After a few moments, one of them shut the door, and she was lost from his sight.
His heart clenched. It felt like an age since he'd seen her, grabbing her keys from a bowl near the door as she pressed a kiss to his cheek before leaving their apartment. But it didn't matter. He wasn't taking any chances. There wasn't anything he wouldn't do to be reunited with Rose Tyler.
Slowly, very carefully, the Doctor withdrew his sonic screwdriver from the front of his coat pocket. With a very determined look on his face, he pointed it towards the door through which Rose had disappeared.
"Can I help you, sir?"
He jumped, his eyes going wide as he stumbled trying to get up from behind the planter that had been hiding him. The receptionist, who was wearing a nameplate that read "Cindy", maintained a pleasant smile, but her eyes were confused. He dusted off his jacket and offered her his most disarming smile.
"Oh, um, hello!" he said, giving her a little wave with one hand while replacing the sonic with the other. "Just -"
He fumbled for the psychic paper (newly replaced by the growing TARDIS), while using his very-impressive brain to come up with a proper cover. John Smith, Lights and Flooring Technician? Janitorial Intern? Decorative Greenery Inspector?
Before he could find the psychic paper, though, he caught sight of the gleaming wedding band on his left hand. Oh. Right! Brilliant. He grinned a grin that was equal parts pleased he'd come up with a clever excuse and delight at remembering that he was, in fact, married. Married to Rose Tyler, which was the best bit.
And that brought him back to his current situation.
"Oh just waiting for the wife," he said, jamming his hands in his pockets and rocking back and forth on his feet. "You know, day with the girls."
The woman gave him a knowing look, as though she'd seen this many a time before. "Poor thing," she said indulgently. "Maybe you'd like to have a look at some of our treatments, while you wait?
Oh, brilliant, wonderful Cindy! She was practically giving him everything he needed in order to get Rose back. He resolved to send her flowers when this was all said and done.
"Hmmm," said the Doctor, walking over to Cindy's receptionist desk, where all the treatments that the Bright Meadows Day Spa had to offer. After spending an appropriate amount of time looking as though he was considering his options, he turned to his new friend.
"What can you tell me about what's happening in there?" he said, gesturing towards The Door.
Thus began the story of how the Doctor's First Annual infiltration of the Third Annual Torchwood Ladies' Retreat.