Maxine Carr either "knew or believed" Ian Huntley had killed the Soham girls when she lied to police on his behalf, the prosecution has alleged.
Ms Carr was accused of 'persistent, devious and detailed lies'
Richard Latham QC, prosecuting, told the Old Bailey "as surely as night follows day" Ms Carr and Mr Huntley were working together to make a false story.
Former school caretaker Mr Huntley, 29, denies killing 10-year-olds Jessica Chapman and Holly Wells, who went missing in the Cambridgeshire town on 4 August last year.
His then girlfriend Ms Carr, 26, denies two charges of assisting an offender and one of conspiring to pervert the course of justice.
Mr Latham said Ms Carr's "lying account persisted through the 10 to 12 days [of the search]... and through interviews with the police".
Ms Carr had explained her lies by saying Mr Huntley had been falsely accused of a crime in the past - before she met him - and she did not want that to happen again, Mr Latham said.
He argued that Ms Carr's motive for telling lies was irrelevant when it came to the charge of perverting the course of justice.
To "stand by your man" is not lawful, Mr Latham said.
The prosecution has pointed to a number of occasions on which Ms Carr let it be thought by police that she had been in Soham on the day of the disappearance, when in fact she was visiting her mother in Grimsby.
"There was not a word of truth in what she told police about her
being in Soham and what happened on that Sunday evening."
'State of mind'
Mr Latham told the jury that Ms Carr could only be found guilty of knowingly assisting an offender if Mr Huntley was convicted, and if they believed Ms Carr knew he was the killer.
"It is her state of mind that matters. She has always strenuously denied that she believed he was the killer," he said.
Mr Latham suggested that "Huntley's diversionary tactics" often occurred just before or after phone conversations with Ms Carr.
He said calls had coincided with Mr Huntley's cleaning of his car and a false report that he had seen a man with a bin liner.
Mr Latham said Ms Carr had told "persistent, devious and detailed lies".
He told the court that while Ms Carr was on remand at Holloway she made two telephone calls to Mr Huntley's mother, Linda Nixon, which were recorded.
In them Ms Carr said she had discussed lying with Mr Huntley on the day after the girls' disappearance, Mr Latham claimed.
She said that Mr Huntley told her the girls had come in their house, one of them had a nosebleed and had been in the bathroom, the other the bedroom, then they left.
The prosecution alleged Ms Carr said that Mr Huntley had called her on the Monday, while she was still in Grimsby, and told her that the girls had come into the house.
Mr Latham said a police search of Mr Huntley's house found "a small
piece of paper" which he said could be described as a "crib card".
He said it was to "sort out the right story on the critical aspect
Written on the paper, Mr Latham said, was "4.55-5pm got in bath" and "5.40-45 dog in", as well as "6.25 come down to put tea on".
The case was adjourned until Monday.