The two boys had just returned from Barker's Eddy where Jem had completed his objective of teaching Dill to swim before he had to return home to Mississippi. They were back on the Meridian highway with little prospect of catching a ride home so late in the afternoon when the two boys saw Atticus approaching, but he failed to see them until they flagged him down. At first, Atticus told the boys they would need to continue their walk home, but
... Jem protested, then pleaded, and Atticus said, "All right, you can come with us if you stay in the car." (Chapter 25)
Atticus and Calpurnia were on their way to the home of Tom Robinson, where he was prepared to deliver the news to Tom's wife of his death. Dill told Scout that when Atticus spoke with Tom's wife, Helen, he never even had to tell her what had happened.
"... she just fell down in the dirt... Like you'd step on an ant." (Chapter 25)