His plan was so watertight that no one could find any flaws in it.

watertight 由 water（水）和 tight（紧密的、牢固的）组成，字面意思是"不透水的"。引申为"无懈可击的、严密的"，形容计划或论证等非常完善，没有任何漏洞或缺陷。