A man left to babysit his partner's nine-month-old baby "violently" shook the boy to death, a court has been told.
Thomas Durkin, 25, allegedly caused fatal injuries to John-Jay Connor, known as JJ, just an hour after he was left alone with the baby on June 14 2011.
Bristol Crown Court heard mother Andrea Glover, 25, waved goodbye to her "giggling" and "happy" son at around noon as she left him with Durkin while she ran errands.
But at 1.05pm, Durkin, of Plymouth, called 999 requesting an ambulance to the home he shared with Miss Glover in Hemyock, Devon.
Paramedics who rushed to the two-bedroomed flat found JJ lifeless and he was taken to hospital to air ambulance.
The court was told JJ was pronounced dead at 2.24pm, with post mortem tests revealing he had suffered internal bleeding, a swollen brain and had bruises to his head and face.
Durkin was later arrested and charged with manslaughter, which he denies.
Prosecuting, Martin Meeke told the court: "On the 14 June 2011 this defendant shook a nine-month-old baby so violently it suffered injuries from which it died.
"Mr Durkin says JJ was in his rocker and the baby smiled at him so he went over, took him out the rocker and placed him on the sofa lying on his chest.
"He says that JJ fell asleep and so did he. When he woke up, he saw JJ was lying on the floor on his back with his eyes half open, making a gurgling noise and not moving."
Durkin later told police he tried to revive JJ by "progressively" shaking him but denies killing the child, Mr Meeke said.
"A fall from the sofa followed by gentle shaking would be extremely unlikely to cause the level of trauma seen," Mr Meeke added.
"Let me make it perfectly clear the prosecution do not accept that Mr Durkin has told the truth about what happened."
The court was told Durkin moved in with Miss Glover, who he had known since they were teenagers, after she separated from JJ's father, John Joseph Connor, 28, in March 2011.
Miss Glover, wearing jeans and a shirt, told the jury JJ was a "content, easy baby" who she felt comfortable leaving with Durkin.
She described how she left her home on June 14 at around midday, with older son Keaton, three.
"He was happy, he was smiling when I left him," Miss Glover said. "He was in his rocking chair.
"As I put Keaton in the car, Tom had got him out of the rocker and was waving at the window when we left."
Miss Glover said she returned to the house a few hours later and found a note on the door telling her JJ was sick and to go to hospital.
During police interview, Durkin claimed he awoke to find JJ had fallen off his chest as they lay on the sofa and was on the floor with "vacant eyes", making a weird noise.
"I shook him a bit, pinched his fingers and slapped his cheeks," Durkin told officers.
"I was really worried about him and getting more and more panicky as it went on."
Durkin told police he splashed cold water on JJ, dunked him in cold bath water to revive him then began shaking him for about 30 seconds.
"I think it started off quite gently, I might have got a bit more progressive, shook him a bit more to see if he would wake up but he wasn't responsive," he said.
"A baby is a fragile thing."
The jury heard a harrowing recording of the 999 call made by Durkin on the day JJ died, in which he attempted to perform CPR on the advice of the operator.
During the 20 minute call, Durkin was heard to say: "He is not floppy, he is going cold. Serioously, I think he might have died. Come on JJ."
Joanna Brown, of South West Ambulance Service, was the first paramedic at the scene at 1.21pm.
She told jurors: "He was lifeless. He was pale, he wasn't showing any signs of life. He was completely unresponsive."
Other paramedics arrived but there was nothing they could do to help JJ and he was airlifted to the Royal Devon and Exeter Hospital, where he was later pronounced dead.
The jury was told pathologists could find no evidence that JJ died of natural causes. Miss Glover insisted bruises on JJ's jaw and head had not been there when she left home.
The trial continues.