Muff Potter was arrested because he was accused of murdering doctor Robinson. Tom Sawyer and Huck Finn witnessed the murder in the churchyard. But it wasn't Muff who killed the doctor, but it was Injun Joe who did it. Muff Potter who was drunk, became a victim of Injun Joe's evil ways, because he placed the murder weapon into Muff's hand while he was unconcious after suffering a hit on the head. Because Tom and Huck swore to each other not to tell anyone what they have seen that night, they hesitate at first to come forward in the trial. But they were aware that an innocent man could be hanged for a murder he didn't commit, so they finally came forward. Injun Joe who was present at the trial, flees out of the courthouse and goes in hiding. Tom Sawyer and Huckleberry Finn know that their lives could be in danger now Injun Joe managed to escape. (thetvdb.com)