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KY 399 begins at an intersection with KY 30 in Vincent, within the north-central part of Owsley County. It travels to the north-northwest and curves to the north-northeast. It begins paralleling and curves to the north-northwest before entering Lee County.[1] KY 399 heads to the north-northeast and curves to the north-northwest. It enters Idamay, where it crosses over Duck Fork and has a brief concurrency with KY 587. The two highways cross over Duck Fork and then split. The highway resumes its north-northwesterly routing, now along the western edge of Daniel Boone National Forest. It crosses over Duck Fork twice and curves to the northwest, where Duck Fork merges into . The highway begins paralleling that creek, curves to the north-northeast, and enters the national forest proper. It curves to the north-northwest and crosses over the Kentucky River on the . It crosses over some railroad tracks of CSX and curves to the west-northwest before entering Heidelberg. There, it curves to the north and leaves the community. The highway heads to the north-northeast and curves to the north-northwest. It heads in an easterly direction and meets its northern terminus, an intersection with KY 52.[2]