The mother/lawyer is questioning her own son in their house. He says he heard the mistress and his father one night on the phone, the mistress telling his father she loved him. The mother/lawyer says "so you followed your dad the next day". The son said he did, all the way to the mistress's house. But you see earlier in the movie that the father didn't go straight to his mistress's house the next day. He went to work first, and after a while when he couldn't get his mistress's office to answer the phone, he left and went to her house. The son is saying he followed his dad from home that morning to the mistress's house, but that's not true.