On Tuesday afternoon, Cambridge Crown Court heard from Tracey Underwood, who is the mother of Luke Holder.
She was staying with her son and his partner, Clare Oakley, at their flat in Cambridge Street, St Neots, for Christmas.
On the evening of December 21, Miss Oakley and Mr Holder, who are due to give evidence today (Wednesday), were at the flat with McWhir and Ms Aldrich for pre-Christmas drinks.
Miss Underwood explained that the four friends were outside on a landing as they were not permitted to smoke in the flat and she was mostly inside with her grandchildren.
She said at some time after 1am she became aware that Mr Holder, Ms Aldrich and McWhir had left the flat. She told the court she went outside to have a cigarette some time later and was aware of Clare on the telephone in some distress.
Mr Gair, for the prosecution, asked her what happened next and she said: "Luke suddenly came running round the corner back towards the flat shouting 'he's killed her, he's killed her'.
She told Mr Gair her son was "hysterical" and she had "never seen him like that before".
The court heard that Miss Oakley made the decision to run to the area of scrub land at Love's Farm where Luke had seen McWhir and Ms Aldridge.
Miss Underwood said her son's clothes were "drenched" and she told him to go inside the flat with the children, while she ran after Miss Oakley.
When they reached the bridge at Love's Way, Miss Oakley spotted Ms Aldridge's handbag on the ground close to the drainage ditch.
Miss Underwood, who was visibly shaken during her testimony, then described how she saw Ms Aldridge's body laying face-up in the water.
"I looked down the bank and saw Marissa in the drainage ditch," she said. "She was under the water. The adrenalin kicked in and I did what I did - I jumped down the bank. She was on her back. I grabbed her, but I knew she was gone - she was blue around the mouth."
She and two other men, who were at the scene, then pulled Ms Aldrich out of the water and dragged her body up the bank.
She told the court one of the men started CPR and then paramedics arrived and took over.
Under cross examination, Miss Underwood was asked about the atmosphere between the friends during the evening and she described is a "jovial".
"Everyone was enjoying themselves. I saw Marissa laughing and joking," she said.
Miss Underwood was also asked by David Whittaker, for the defence, about how she entered the drainage ditch to help Marissa.
Mr Whittaker said: "You were unconcerned for your own safety, you jumped in and effectively dragged her out of the water.
"Yes," she replied. It was the adrenalin and human nature kicked in."
Miss Underwood described the water as waist-deep and freezing cold and said at one point her shoe became stuck in the silt at the bottom of the water.
Mr Whittaker said: "So you could have easily slipped down there?" and she replied: "Yes, I could have."
Robert McWhir, 26, denies murdering Marissa Aldridge on December 22, 2018.
The trial continues.