Becky Watts was murdered by her stepbrother and his girlfriend "acting together", a court was told.
The 16-year-old was killed in the bedroom of her home in Bristol by Nathan Matthews, 28, and Shauna Hoare, 21. The prosecution claim that after suffocating her, the pair dismembered her body with a knife and a power saw.
Her dismembered body was found wrapped up in clingfilm and hidden in suitcases and boxes in March.
Matthews has admitted manslaughter but denies murder and conspiracy to kidnap.
Hoare has denied murder, conspiracy to kidnap, perverting the course of justice, preventing a lawful burial and possessing a prohibited weapon, including murder, claiming she was unaware of what was going on at the time.
Prosecutor William Mousley QC said the defence was "ridiculous" and insisted the killing was "a case of two people very close, very together, acting together", The Mirror reports.
He said: "You can be sure this was no accident when [Matthews] killed her - this was not something that just went wrong.
"You can be sure that Nathan Matthews is guilty of murder and [Hoare's] proximity, her involvement and her behaviour and the sheer implausibility of her version of events on the evidence which is available and the ridiculous concept that she was in blissful ignorance of what was happening at the time."
The court also heard how Becky's boyfriend, Luke Oberhansli, now 18, went to her home in St George after she stopped replying to his text messages.
Shauna Hoare opened the door and she spoke to Becky's grandmother, Anjie Galsworthy. Both women then went to check the teenager's room, leaving Mr Oberhansli waiting on the doorstep.
Prosecutors allege Becky was already dead by this time, having been murdered by Hoare and her boyfriend in her bedroom.
Yesterday, the court heard Matthews and Hoare shared an "unnatural interest in attractive teenage females".
Mr Mousley told jurors two stun guns had been found at the home of Matthews and Hoare, which he said "may have been intended for use in the planned kidnap".
The day after the alleged murder, one of them searched "Do you want to hide a body?" on YouTube, the court was told.
The trial continues.
