Warning: Wikipedia contains spoilers!
Whilst inbetween changing trains in Philadelphia, an Amish widow's son witnesses a murder in the train station's toilets. It turns out that a police officer is the murderer, and Philadelphia detective John Book quickly decides to hide the mother and child for their own protection. But when John's partner is murdered by another crooked cop looking for the boy, John himself must go into hiding.
The only place he can think of is the farm run by the woman's family, where he takes her and boy. Forced to stay because he has been shot, he adopts Amish dress, participates in the community, and begins to fall in love with the widow he is protecting.
But upon a visit to town to use a public telephone, he gets into a fight with some youths who are abusing some Amish -- blowing his cover. The corrupt police from the city hear of this event which is uncharacteristic of the Amish, and come looking for him.
The movie portrays the strong contrasts between the debased and unempathetic urbanised world John Book lives in, and the simpler, ruralistic lifestyle of the Amish, which is pre-industrial revolution-European in nature.
Although the train station is not identified in the film, the station scenes (including the murder) were filmed in Philadelphia's 30th Street Station.