A six week-old baby was beaten to death with a shoe and a plastic bottle, a court has heard.
Michael John Pearce is on trial at Newport Crown Court charged with the murder and manslaughter of little Alfie Sullock.
The 33-year-old, who denies both counts, was looking after the newborn for the baby's mother Donna Sullock while she went out on her first night out since giving birth.
A jury heard that four minutes before Pearce made a 999 call, he had texted Ms Sullock to reassure her son was doing fine.
But when paramedics arrived they found Alfie blue and lifeless - and noticed bruising to his face and chest.
Following a CT scan, doctors discovered the baby had suffered a brain haemorrhage and said there was no hope of him surviving.
Opening the case for the Crown prosecuting counsel Michael Mather-Lees QC said Alfie was gratuitously injured while in the sole care of the defendant.
He said: "This was not a moment of exasperation that can happen with a screaming child.
"This was a baby repeatedly beaten with objects.
"This is not a case of a baby developing sudden death syndrome.
"This was a child who was badly beaten and as a result the baby dies. He (the defendant) killed him.
"What possible explanation can somebody have to beat a defenceless baby and to cause them really serious harm?"
Alfie was born at the University Hospital of Wales (UHW), Cardiff, on July 6 last year.
The court heard the child was healthy and that his mother's pregnancy and labour had been uneventful.
A jury also was told Ms Sullock, who lived in Fairwater, Cardiff, discovered she had become pregnant while a holiday rep in Crete.
She returned home to the UK, and six months into her pregnancy she became friends with the defendant.
Mr Mather-Lees told the jury although the pair went on a number of "dates", divorcee and father-of-one Pearce was becoming a "little bit presumptious to the state of their relationship" and displayed "obsessive behaviour" towards her.
On August 16, Ms Sullock travelled from her home in the Welsh capital to Pearce's home in Nelson, Caerphilly.
That day, the defendant asked Ms Sullock would he give her a baby - to which she replied "no".
Before going out, she fed, winded and changed her son - who the Crown say was healthy and breathing normally.
Mr Mather-Lees added: "This was the first time Alfie's mother had left him with anyone else.
"The baby was unbruised when his mother left.
"During the course of the evening various text messages were exchanged.
"At 8.16pm she received a picture message and asked if the baby had been crying as his face appeared red.
"The defendant said the baby had wind but was fine."
Pearce then sent another picture message around 15 minutes later and at 8.43pm told Ms Sullock "you can trust me you know".
He also told her that "everything is absolutely fine" before saying he was watching TV.
But at 9.11pm, Pearce dialled 999.
The call, which lasted around seven minutes, was played to the jury in full.
When asked by the operator if he had seen what happened, the defendant replied that he had given the baby a bottle and then the child had stopped breathing.
He said the baby felt cold to touch, before he was instructed to carry out mouth-to-mouth resuscitation.
Mr Mather-Lees added: "Plainly he says he had no idea what happened. But of course he did.
"Children do not go cold instantly."
The jury was shown photographs detailing 10 facial injuries to the child, as well as bruising to his abdomen.
Mr Mather-Lees said the "distinct" marks on the defendant's shoes "precisely" matched injuries on the child's face.
Alfie was first taken to the Prince Charles Hospital in Merthyr Tydfil before being transferred to UHW.
Doctors found the newborn had been "gratuitously" injured and the following day - on August 17 - he was found to have no brain function.
On August 20, life support was withdrawn and Alfie died, the court heard.
A Home Office pathologist gave a provisional cause of death as "blunt trauma head injury".
The jury heard Pearce was interviewed by police extensively and gave "no explanation for the injuries" Alfie sustained.
Before completing the opening of the Crown's case, Mr Mather-Lees told the jury: "All the evidence points in one direction".
The trial, which is expected to last three weeks, will continue tomorrow.