Anne is shelling peas when Marilla walks in and asks if Anne took her amethyst brooch.
Anne admits to trying it on, but she says she returned it to Marilla's bureau afterward.
Marilla re-checks the bureau and her whole bedroom, but no brooch. So she returns to Anne and accuses her of lying.
Anne refuses to confess, so she's sent to her room. It's the same punishment as earlier: stay in your room until you do the thing. In this case, confess.
What bugs Marilla more than a missing brooch is the idea that there's a child in her house who lies and can't be trusted. She tells as much to Matthew the next morning, but Matthew's completely willing to stay out of this one.
The next day, Marilla searches for the brooch and Anne stays in her room. In the evening, Marilla tells Anne she can't go to the picnic until she confesses.
So the next morning, Anne "confesses." She tells Marilla she took the brooch and accidentally dropped it in the Lake of Shining Waters. But she says it all without any feeling, like she's reciting something.
A super-annoyed Marilla forbids Anne from going to the picnic. Matthew tries to persuade her otherwise, but Marilla's foot is down.
But later, when Marilla goes to mend her black shawl, she finds the brooch caught in it.
Anne admits she made up the story of dropping it into the lake so she could go to the picnic. Marilla apologizes and sends Anne there with some food.
The picnic doesn't disappoint. Afterward, Anne tells Marilla that she had tea, was rowed around the lake, and tasted ice cream.
Marilla tells Matthew that she thinks Anne will turn out all right.