I may have to work late, in which case I'll call you.
翻译:
我可能得工作到很晚，如果是那样的话，我会给你打电话。
知识点解析:
非限制性定语从句。in which case 相当于 and if that happens，引导非限制性定语从句，修饰前面整个句子。