How did Nikolay and Princess Marya first meet?

They meet at a soirée in Voronezh.

I am not looking into the book for the details, but they meet when Prince Bolkansky just died and despite Andrei's warnings to his sister and father, the household had not successfully left the area.  Russian troops wereretreating  just ahead of Napolean's; Smolensk had surrendered.   Prince Bolkansky refused to leave Bald Hill, and Masha stayed with him out of concern for his health.  When the prince had a stroke, the household attempted to move to Moscow by first going to Bogucharovo (Prince Andrei's estate- I do not know the geography involved), but the prince died in Bogucharovo.  When Marya attempted to continue to Moscow, the serfs and peasants belonging to the estate were hostile and caused trouble (said the horses/carta were unavailable and the like).  Just around this point, Bogucharovo was mid-way between the Russian retreating army and NApolean's forces.  Rostov, with maybe 2 other captains (or whatever the rank), rode out to Bogucharovo to look for supplies and found Princess Marya in her plight.  Nicolai succeeded in rescuing the household and securing their retreat to Moscow.