Witness (1985)

A big city cop. A small country boy. They have nothing in common... but a murder.

A sheltered Amish child is the sole witness of a brutal murder in a restroom at a Philadelphia train station, and he must be protected. The assignment falls to a taciturn detective who goes undercover in a Pennsylvania Dutch community. On the farm, he slowly assimilates despite his urban grit, and forges a romantic bond with the child's beautiful mother.