Becky Watts was murdered and dismembered after her stepbrother and his girlfriend kidnapped her for a sexual motive, a jury heard.
The 16-year-old was allegedly killed in her bedroom at her home in Crown Hill, Bristol, by Nathan Matthews, 28, and Shauna Hoare, 21, on February 19 this year.
Matthews and Hoare deny murder.
The trial was told Matthews admitted killing Becky, stating he tried to kidnap and imprison her. He told the court he strangled her and acted alone.
Bristol Crown Court heard she was suffocated "despite her fighting for her life" after the couple arrived at the house that morning and placed her in the boot of the couple's Vauxhall Zafira, the Press Association reports.
Matthews and Hoare remained inside the property until about 6pm – during which time family members arrived – when they drove to their home address in Cotton Mill Lane, Bristol.
That evening, the couple ordered a Chinese meal from a takeaway from where Matthews' worked and later attempted to call a pizza company, as well as watching television.
The following day, Matthews bought two bottles of drain cleaner from a superstore before driving to B&Q, where he purchased a circular power saw, gloves, face masks and goggles.
CCTV footage from B&Q shows Matthews querying the price of the power saw, which was later used to dismember Becky's body, the court heard.
Becky's body parts were then stored in a shed at a house 80 metres away from where Matthews and Hoare lived.
William Mousley QC, prosecuting, told the jury of 11 women and one man that Matthews and Hoare, who was pregnant at the time, hatched a plot to kidnap Becky.
"She was suffocated, despite her fighting for her life. There followed a deliberate, carefully planned and grotesquely executed plan to cover up her killing," he said.
"Following her removal from her home, over the course of the next few days, her body was cut up with a knife and a power saw, the parts carefully packaged and then moved to another address to prevent them being found and lawfully buried, where they were intended to be stored probably temporarily until a final solution could be found.
"Nathan Matthews and Shauna Hoare were responsible for her death and that cover-up, assisted by four others, acting together, including James Ireland and Donovan Demetrius, who, to varying degrees over a period of a week, helped in the hiding of her disguised remains in the knowledge that she had been killed or that some other significant offence had been committed by Nathan Matthews and intending that it should prevent the apprehension or prosecution of Nathan Matthews and Shauna Hoare.
"Her death was the result of a plan by Nathan Matthews and Shauna Hoare to kidnap Rebecca Watts where it seems that items were taken to Rebecca Watts's home to carry it out.
"In addition to their apparent dislike of Becky, there is good reason to believe there was also a sexual motive behind the scheme, arising from a shared unnatural interest in attractive teenage females, and that a foreseeable conclusion to it being carried out would either be Becky's death or serious injury."
Becky's mother, Tanya Watts, fled the court room in tears as the case was opened to the jury.
Her father, Darren Galsworthy, listened intently to proceedings, while other family members sat in the public gallery of court room two, wept and stared at the dock where the defendants sat.
The court heard Becky was left alone at the home where she lived with her father Mr Galsworthy and stepmother Anjie - Matthews' mother - on February 19.
Matthews and Hoare, who had been in a relationship for six years, arranged for a key to be left outside the property so they could let themselves in, it is alleged.
Just after 10.50am, Matthews and Hoare left their home before purchasing batteries at a Tesco supermarket to use for stun guns then drove to Crown Hill, the jury heard.
At 11.03am, Becky sent her final text to a friend.
Mrs Galsworthy returned home from a hospital appointment at 12.45pm and a friend of Becky's later visited the property after failing to get hold of her.
"Shauna asked Anjie and Anjie said she didn't think that Becky was there but would let Becky know that they had called," Mr Mousley said.
"It was about that time that Darren Galsworthy returned home from work."
Matthews and Hoare then left the property with Becky's body allegedly in the boot of their car, with a child sitting in a car seat in the rear of the vehicle.
At 12.20pm the next day, Matthews bought drain cleaner from a superstore before driving to B&Q in Horfield, where he arrived at 12.45pm, buying a circular power saw, gloves, face masks and goggles.
The court heard Becky was reported missing that day by her father and police launched a "full scale investigation", Mr Mousley said.
He said both Matthews and Hoare lied to the police about visiting Becky's home on the morning of February 19.
When forensic experts discovered Matthews' fingerprints in Becky's blood on door frames outside her bedroom, both he and Hoare were arrested on suspicion of kidnap.
Her body parts were discovered in a garden shed in Barton Court - 80 metres from where Matthews and Hoare lived - on March 2.
Another couple, who were accused of storing the dismembered body in their shed, have pleaded guilty to assisting of an offender, it can now be reported. They claimed they did not know it was her body.
Karl Demetrius, 29, and his girlfriend, Jaydene Parsons, 23, admitted the charge at an earlier hearing at Bristol Crown Court, the BBC reports.
Donovan Demetrius - Karl's twin brother - and James Ireland, 23, are also charged with assisting an offender, which they both deny.
The trial continues.