Seventeen officers are participating in the steeplechase (which is an obstacle course over three miles).
Vronsky and Frou Frou are absolutely splendid. They overtake Gladiator towards the end of the race and are in the lead. They have one tough obstacle left, which Frou Frou clears easily. Vronsky is ecstatic.
They head to the finish line, pausing to clear a simple water jump.
Essentially, Vronsky messes up during the jump. Instead of keeping his motion in line with the horse, he sits back in the saddle.
The two fall. Gladiator overtakes them.
Vronsky tries to urge Frou Frou to get up, but she just lies there looking at him.
Her spine is broken.
Vronsky is uninjured, and angry at himself because of it.
Doctors, attendants, friends, and other people run over.
They have to shoot Frou Frou in order to put her out of her suffering.
Frou Frou's injury and resulting death is Vronsky's fault. He know that he is to blame and is miserable. This is the first time that he's really experienced misfortune in his life.
For a long time, this is the worst memory of Vronsky's life.