The frame story: A young disillusioned Frenchman, René, has joined an Indian tribe and married a woman named Céluta. On a hunting expedition, one moonlit night, René asks Chactas, the old man who adopted him, to relate the story of his life.
At the age of seventeen, the Natchez Chactas loses his father during a battle against the Muscogees. He flees to St Augustine, Florida, where he is raised in the household of the Spaniard Lopez. After 2½ years, he sets out for home, but is captured by the Muscogees and Seminoles. The chief Simagan sentences him to be burnt in their village.
The women take pity on him during the weeks of travel, and each night bring him gifts. Atala, the half-caste Christian daughter of Simagan, tries in vain to help him escape. On arrival at Apalachucla, his bonds are loosed and he is saved from death by her intervention. They run away and roam the wilderness for 27 days before being caught in a huge storm. While they are sheltering, Atala tells Chactas that her father was Lopez, and he realises that she is the daughter of his erstwhile benefactor.
Lightning strikes a tree close by, and they run at random before hearing a church bell. Encountering a dog, they are met by its owner, Père Aubry, and he leads them through the storm to his idyllic mission. Aubry's kindness and force of personality impress Chactas greatly.
Atala falls in love with Chactas, but cannot marry him as she has taken a vow of chastity. In despair she takes poison. Aubry assumes that she is merely ill, but in the presence of Chactas she reveals what she has done, and Chactas is filled with anger until the missionary tells them that in fact Christianity permits the renunciation of vows. They tend her, but she dies, and the day after the funeral, Chactas takes Aubry's advice and leaves the mission.
In an epilogue it is revealed that Aubry was later killed by Cherokees, and that, according to Chactas's granddaughter, neither René nor the aged Chactas survived a massacre during an uprising. The full account of Chactas's wanderings after Atala's death, in Les Natchez, gives a somewhat different version of their fates.

Question: Why does Atala take poison?
Answer: Because her chastity vows prevent her from marrying Chactas
Alternative answer: because she can't marry Chactas