As well as using ‘where’ to refer to places, it can also be used to refer to points in time at which something happens. That was where the phone rang = the point in time at which the phone rang I was walking in the countryside and just after I passed two friends was where the rain started. (It’s the same really as using ‘when’)

'Whence' is a good alternative to 'where' in this sentence. Where is still used correctly.