Under the Gothic spires of St John's College, women in gowns and men in dinner suits danced into the night to celebrate the end of the year.
They ate hamburgers and gelato, and lined up at an open bar serving champagne, beer and cocktails.
Jean Claude Perrottet, the younger brother of NSW Treasurer Dominic Perrottet, was at the ball as a guest of a college resident, and spent the evening drinking, dancing, and having a political argument with "a leftie".
A 19-year-old woman got ready with friends, drank champagne, and was spotted running into the crowd and dancing like "crazy" at the end of the night.
Mr Perrottet and the woman met and kissed on the dance floor at the after-party, then moved to a canopy of trees near an area called the Lemon Grove.
What happened next, in the early hours of October 18, 2015, changed everything.
The woman remembered being raped, repeatedly telling Mr Perrottet to stop and get off her. Mr Perrottet remembered some consensual sex acts, before the woman sat up, said she did not want to continue, and they stopped.
In the NSW District Court on Thursday, a jury of seven women and five men took less than two hours to find Mr Perrottet not guilty of three counts of sexual assault.
The two-week trial was complex, dealing with alcohol, memory blackouts, inconsistent statements, religion, and youth.
The defence case was a textbook example of the principles of "beyond reasonable doubt" and the burden of proof.
The woman said she drank more alcohol than she usually would, so was unsure of the chronology of some key events, and could not remember others.
Eloquent and unfazed under cross-examination, the woman was adamant she told Mr Perrottet to stop, and had clear and distressing memories of those moments.
But Mr Perrottet's softly-spoken defence barrister, Alissa Moen, pointed to the woman's varying statements to police and doctors, and how heavy drinking had affected the long-term memory of all those involved, including Mr Perrottet.
Forensic evidence that would support the woman's allegations was absent, she said.
It was possible the woman's memory was "contaminated" by one of her friends, who saw her bedraggled state at the end of the night and told her: "You were raped."
"If you simply don't know what happened beyond reasonable doubt, that would lead you to not being satisfied of any of the counts," Ms Moen said.
Mr Perrottet told court he was so drunk that he dozed off during the encounter, and said he did not force her to do anything, nor did she object.
When she suddenly sat up and said "I can't do that", Mr Perrottet said he did not want to have sex with her.
Mr Perrottet, who comes from a large family belonging to the conservative Catholic order Opus Dei, later told police there was no way they had sexual intercourse. "It's against my religion," he said.
"He's a young man ... with quite an unusual level of empathy, that's just part of who he is," Ms Moen said.
"When you look at what the complainant actually accuses him of, particularly what she's alleging involving two to three minutes where she's making it quite clear she's suffering … in an act that would be incredibly barbaric and distressing … my submission is it's very, very improbable."