Until that moment, Mary thought her uncle and his friends were smugglers and were doing things against the law.

against 是介词，意思是"反对、违反"。 the 是定冠词，表示特指某事物。 law 是名词，意思是"法律"。 against the law 是一个固定短语，意思是"违法的"，指行为违反了法律规定。