The Doppler effect of a measured set of footsteps told her someone was coming toward her new prison cell – and their light tapping told her that they were not the footsteps of any of the guards that she'd met so far. A soft knock at the door, and a well modulated alto voice asked: "Doctor Song?"

She looked up at the door and saw a pair of piercing dark blue eyes staring in through the window. The door swung open and there stood a tall woman with straight ash blond hair falling past her shoulders. River knew her, knew the white jacket and skirt, the uniform of the Galactic Justice Department, so reminiscent of late 20th century businesswear. Humans in this time period were going through a nostalgic period, apparently.

"What do you want?" River demanded. This woman had been the one prosecuting her, gunning for her execution – then abruptly, when River's defender pointed out that the victim's body was missing, asked for a sidebar conference with the defender and the judge. The conference went on for two hours, after which River was abruptly convicted of attempted murder and sentenced to life in prison at the Stormcage Detention Center, with possibility of pardon.

"I thought you'd want to know what happened," the Prosecutor said.

"I thought I couldn't be told until after I was released," River snarled. "They wouldn't even tell me your name."

"The rules can be bent somewhat," the prosecutor said, smiling wryly. "I'll tell you my name, if that makes you feel better. I am Thérèse Sigman." When River favored the woman with a sceptical glare, she reached into her jacket pocket and pulled out and ID. "Here, I'll prove it."

The ID matched. River finally handed it back, but she didn't trust herself to speak. She remembered the Doctor telling her about a prosecutor called the Valeyard, and River had come to feel the same way about this woman – Prosecutor 68445, she was called in the court records; just "Madame Prosecutor" in court.

"I learned that the Doctor knew you had no intention to shoot him, that it was an accident."

"Of course it was!" River snapped. "He was the best man I had ever known – not just in terms of intelligence, strength, stamina, I could go on for days – he was always trying to do the right thing. And he usually did. I loved him. He was my husband. ... I would just as soon have blown my own brains out."

"He was well aware of all that," Sigman replied. "Here's what happened: He heard the gun go off as he was standing up and he tried to throw himself on his face. Another person might not have been able to react quickly enough. Plus, an analysis of the crime scene with information provided by his TARDIS showed that if he hadn't, he would have been killed instantly.

"As it was, his brains were leaking out of a hole in the side of his head – but he had time to start the regeneration, and he had time to formulate a plan. Or perhaps 'desperately throw together a half-assed idea' is a better description."

"Which was?"

Sigman smiled ruefully. "The idea was to regenerate as a female – specifically as a duplicate or near-duplicate of you, Dr. Song. Of course, that meant he had to exercise a degree of control over the process that he had never attempted before. You've noticed how different his past regenerations are from each other, I'm sure." Sigman shifted her weight with a slight wince, as if silently complaining about her high-heeled shoes. "It wouldn't have fooled anyone in the long term, of course, but it would have allowed you time enough to escape. The Celosians knew you were a human. They knew enough about Time Lord anatomy to be able to identify one from X-rays, unlike certain other species at certain other points in time" – she smirked – "and they would be able to detect artron energy residue from a recent regeneration. Once they released the Doctor, he – by now, she – would have tracked you down, explained what happened and found a way to keep you from being captured."

River grabbed Sigman by the wrist; the prosecutor didn't resist. River could tell, without counting, that this woman's pulse was dangerously fast – or would have been for a human. First, she was vastly relieved – then she was incredibly angry.

"If you knew I was innocent, why did you prosecute me? Were you that angry at me?"

"I had to, River! I don't know if you caught that during the trial, but they can execute people for causing a Time Lord to regenerate prematurely. They generally don't, but they really have something against people using illegal projectile weapons. I got to the TARDIS before I passed out, and the TARDIS left. The regeneration actually happened in Vortex, and my recovery took much longer than I had expected. There were ... complications that may or may not have been related to forcing a cross-gender regeneration. When I was back to myself, I went back about six months, hacked into their system as a new prosecutor and made sure I got assigned to your case. I did that so I could put in the plea for leniency that commuted your sentence to life."

River dropped her wrist, turned away, then turned back. "You shouldn't have bothered," she said. "This is the rest of my life. What kind of life is this going to be?"

"I had to, River. Our timelines are woven together, but we're travelling in opposite directions. I've been dealing with you since my tenth regeneration. You escape on occasion, but you promise me that you will return, and you do. If the Celosians had executed you, then my timeline – and who knows what else – would have changed."

River sat down on her cot and tried to pull her head together. The Doctor leaned against the wall and watched silently. Finally she sat down next to River and put one arm around her shoulders, but River shook her off.

"River, you're still alive. You might even get a pardon. Can't you forgive me? This is the best I could do – and in case you've forgotten, that's a damn sight better than most living creatures could do."

"I know that! And ... I know you're right. It isn't that I'm stuck in prison."

"Then what's your problem?" the Doctor half-shouted.

"Look ... you may be the Doctor, but you're not my Doctor anymore."

"And why is – oh," she said, as River's eyes tracked quickly down her body. The Doctor raised her eyebrows. "Really," she said, as if assessing a strange quirk in a law of physics. "Kind of surprising, given how most other humans in your time view such things."

"I'm a bit of a throwback in some ways."

The Doctor looked as if she were going to cry for a fleeting moment. Instead, she set her jaw and nodded. "I would get involved with the only entirely heterosexual woman in the 51st century," she muttered. "Well, you're still alive." Then she smiled, and her eyes even twinkled, but there was something angry underlying it all. "And don't worry. I'll be back … in a form you'll find more pleasing, no doubt."

River could almost taste the bitterness in the Doctor's voice, even as she reached out to her left, opened a door in midair, stepped through it, gave a jaunty salute and, with a receding Doppler effect of the TARDIS' grinding engines, was gone.