Ah, it doesn't explain very well, sorry! Ros is lead officer on a number of murder investigations. There was a "live" abduction in similar circumstances (balaclava man!) which ended up (after a police chase) in a burning house with the victim tied up inside. The house was where the young lad in prison lived (his nan's house I think), hence getting him for the crimes.
Ros was in charge of the scene and went in to rescue the victim (the waitress they have how got for Tim's murder). Tim found forensic evidence that jewellery linking the house to the previous incidents could have been planted, including an additional footprint at the scene. He tried to go to Ros, who ignored him, so he went to AC12.
She was none too impressed about being reported to AC12. I hope I have remembered all that right!