He knew this was coming. It was impossible to avoid it, it need to be did not mean he like it. But what did he expect, he's a time lord living among humans.
It is pretty easy to pretend to be human if he just fudged some miner information like the fact he had two hearts, with his mothers sonic screw driver. He had been doing it sense he was eight. He hated to have to lie to his team mates but he had to. What could he say that he was an alien from space who's mother used the time streams running wild during the war to propel a small ship to earth to save her unborn son. That seeing her home planet get destroyed in front of her made her go in insane that her paranoia was justified that there could have been Dalek after her or if the government ever found out that they were aliens they may vary well be in danger. They had many fights over this but she always let him make his own decision leaving him with this one warning "One day you will have to regenerate and leave this life behind you"
That day had finally come he had to regenerate there was no way he could avoid it. He got shot in the chest the only way he could live without regenerating would be to have open heart surgery but, he couldn't because he had two hearts and could not let any one find out. So he did what was needed to be done no matter how much it killed him he faked his own death.
The case was a simple one until it wasn't, the unsub had inserted himself in to the investigation. He decided that Reid was the major threat. He tricked Reid in to going to the crime scene or he thought he did. Reid figured It out and left a note to warn the team, He hoped that would get there in time. They didn't.
It was a stand off in the forest. Both of them had there guns pointed at the other and fired one bullet hit the head the other the chest. Reid fell to his knees clutching his chest trying to stop the bleeding in his chest, the unsub lay dead in front of him. It was over he had to die there was no team to come save him the only way to survive was regenerating. He needed to regenerate he could feel the heat building up in his body Something he had not felt sense he got shot in the neck about a year ago.
He had to leave before the team came.
He could not let them see him regenerate.
He staggered to the river a few feet away hidden by tree's when the fire inside engulfed him. It burned every single cell in his body made him scream in agony he felt the bullet being expelled from his body as the skin and organs repaired themselves bones re-knitted and reshaped everything was changing that was just a painful as the burning.
When it finally stopped he staggered forward and stood up to his new now stood at he know stood a five foot six.a little bitt shorter then before but not to bad. He went throw all the basics, two legs, two arm, nose, eyes, gender, all the basics. He now had long red hair pale skin and he didn't know his eye color yet.
"Kay new look" he said slowly "Now I need a new identity. One question where am I?" He took a step forward and feel in to the river.