Ralph will have a narrow escape: he is saved by the arrival of a ship which has at last come to rescue them all.

a 是不定冠词，表示"一个"或"一次"。 narrow 的本义是"狭窄的"，在这里引申为"勉强、差一点就失败的"。 escape 的意思是"逃脱、逃离"。 a narrow escape 是一个固定短语，意思是"侥幸逃脱"或"死里逃生"，指在危险情况下勉强成功逃脱。