The Doctor found her in a party on Dreyla 18. He'd been running around, tinkering with little bells and nodules in order to avoid tripping the alarm system, occasionally chatting with some random bloke in order to blend in. It hadn't worked too well, but he hadn't had anything more than the usual strange look. The Frewtins weren't supposed to know that he was there, not The Doctor with all his capitals, but a strange man having strange conversations wasn't a big tip off just yet.
He'd ducked under a table at the last possible moment to avoid being seen by a guard, only to find that the table was occupied.
River Song was curled up, her skirts taking up far more room than she'd originally intended, her finger pressed to her lips urgently.
The Doctor nodded, scuttling closer so that the tablecloth fell around him. In the dark, he could still make out her smile and her arched eyebrow.
"I found a thing," he whispered.
Smiling, River nodded, taking the finger away from her lips.
They both paused, listening for the heavy sounds of the guardsman. There weren't any. The Doctor moved to press an ear to the ground, looking out from underneath the tablecloth. No one close by. He sat back up and motioned for her to leave. She did just that, and how she managed to get out from under the table with all the skirts he did not know, but she was all grace and finesse.
When he stood back up, he grinned at her. She was wearing his favorite dress, the one he'd taken her ice skating in for her birthday.
"Have we done the Frost Fair?" he asked, offering her his arm.
"So you recognize the dress?"
She led him over to the dance floor. She lifted the train of her skirt in one hand and turned to face him, placing the other hand on his shoulder. He gripped her waist tightly and they set off dancing.
River danced smoothly, like one who had years of practice under her belt. The Doctor danced like one who had to think about his every step, but he tried his best not to show it. She pretended not to notice the small fumble for grip when he spun her. He let her. They continued to dance in easy silence for a few bars, letting music guide their actions.
"Do you see it?"
"Over there, underneath the painting of the Frewtin Civil War general."
"Oh! That 'it'! I disarmed that a few moments before I found you."
"It's blinking red, Doctor."
"… It wasn't doing that a few moments before, was it?"
"I think you may have been found out."
"… It is blinking red, isn't it?"
"The guards are all watching us now. Shall we at least put on a show?"
The two separated in a blink, each moving in the exact opposite direction. River pulled a gun from her sleeve and shot at the painting, letting it fall to the floor in a crash of sparks and coal. The partygoers jumped again when they heard the high pitched sonic as it opened a door. The Frewtin police force scrambled to chase after them both, hopping over tables and chairs and guests. After minutes of running and shooting and spinning, River and the Doctor both made it back to the TARDIS safely. Safely enough, anyway: the back of River's dress had gotten a bit singed by a stray blast and the Doctor had punch dripping from his hair.
As the Doctor began to plot coordinates, River began to spin the time constraining loop and gave the transducer a kick. Together, they piloted the TARDIS out of there, letting her spin off into the vortex.
With time to stop and chat, the Doctor made his way over to River, watching over her shoulder as she pulled the space reflexer lever down to the red.
"Where did you learn how to dance, River?"
"Not from you, if that's what you think."
"I did not think that," he replied, indignant. River rolled her eyes, watching from the corner of her eye as he crossed his arms melodramatically. "But… where? You're good at it."
"Well thank you, sweetie."
"Where? Do you remember?"
"Of course I remember. I learned from my Mum and Dad."
The Doctor cracked a smile.
"Then tell me about it."