Mischa Barton was not in her right mind after she says someone spiked her drink with GHB ... and video of the incident shows it was a dire situation.
The video -- shot Thursday before she voluntarily went to the hospital -- shows the 31-year-old former 'O.C.' star rambling, then losing her grip on her fence and falling to the ground.
Barton told People this was all a reaction to the so-called "date rape" drug -- which someone dropped in her drink while she was celebrating her birthday Wednesday night.