Can anyone I.D. this plant that appeared in our woodland garden in
central North Carolina?

http://nickplummer.home.att.net/unk2.jpg -- Flowers
http://nickplummer.home.att.net/unk3.jpg -- leaf

The plant produced a rosette of leaves last year, vanished during the
winter, and reappeared with a 3-4 foot stem this year. It gets some
sun around mid-day but is otherwise shaded by mature deciduous trees.
I have no idea whether it is a wildflower or an escaped garden
variety.

I suspect it might be a Campanula or something similar, but I can't
find anything that matches exactly.

Thanks.