S1E34: Everyone's Gathered! Usopp Speaks the Truth About Nami!

Johnny tells the others about Nami killing Usopp, and Luffy does not believe them until Nami appears herself and confirms it. She tells them to stay out of her business and leave without her, but Luffy stubbornly decides to stay on the island. Usopp returns, revealing that Nami only pretended to kill him, and Nami's sister, Nojiko, starts to explain why Nami is acting the way she is.