Bob Ewell's testimony comes directly after that of Heck Tate. Bob is well known for being a drunk, which has caused his family to fall on hard times. It's likely that the only reason Bob's testimony is taken seriously at all is because he has white skin and is claiming that a black man has committed a heinous crime.
When his testimony begins, Bob provides a sensationalized account of the rape that he alleges to have been witness to—his daughter Mayella being raped by Tom Robinson. Thanks to the vile language that he uses and his generally unpleasant demeanor, Bob causes a scene in the courtroom, and the judge had his work cut out for him to restore order to the court.
Bob's testimony—crassly delivered—explains that while walking home from the woods one evening, he happened upon Tom Robinson raping Mayella. According to Bob, Tom then ran out after having been caught in the act.
Extreme doubt, however, is cast on Bob's story when he is cross-examined by Atticus Finch.
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