When Cayuse children become sick, a Medicine Man who cannot cure them is banished, as is the tribal custom (banishment or worse). Marcus, also a medical doctor, comes forward and helps the children get well.
Chief Timothy becomes the first Cayuse to convert to Christianity (scene 9). But Joe Lewis warns the Cayuse that Marcus is a fraud, that the true "white religion" is Catholic, as espoused by priests who have arrived in the area for the first time. Marcus fears that their arrival will make the missionary task even harder (scene 10).
Cayuse children are sick again. This time, Marcus cannot help them. White children recover from this measles epidemic but Cayuse children are dying. Some chiefs want to hold Marcus responsible as a medicine man giving bad medicine.
The chief woos Lorinda, treating her with respect, but fails to win her heart. He finally lets her return to her people. In the meantime, the chiefs are captured and convicted of murdering the missionaries.