[误] Let\'s discuss this matter in details.

[正] Let\'s discuss this matter in detail.

注：in detail 一起相当于一个副词，指“详细地”;而 in details 中，details 是名词，指“细节”。所以，上面的句子也可以表达为 Let\'s discuss the details of this matter 或 Let\'s go/enter into details。