Alisha is driving along a narrow forest highway with Sosha as her passenger. Sosha, it turns out, is desperate to pee. The road, however, is far too narrow and lacks any turn outs. There is no place for Alisha to safely stop, meaning Sosha must try and hold on.
Sosha begs Alisha to stop as soon as possible, clearly on the verge of having an accident. Alisha is sympathetic, and promises that she will stop as soon as she can, but there is no shoulder and no turn outs, so currently stopping is impossible. With an achingly full bladder, Sosha is forced to contort herself into all manner of positions, every moment she is able to hold on is a battle won.
Finally, Alisha spots a small turn out. She pulls over and stops the car. Sosha flings the door open and bursts from the vehicle, but as she does so her bladder lets go. At her limit, Sosha is unable to hold back. If they would have stopped a minute earlier, things would have been okay. As Sosha leaps out of the car she pees, completely soaking her pants as Alisha watches.
Alisha is a bit amused, but also sympathetic. Sosha is, of course, completely humiliated. There are spare clothes in the back of the car, so Sosha goes to change. We get to watch as she digs through the back of the car, wearing her wet pants, then undresses outside and puts on clean clothes.