有個後門 There Must Be a Back Door
行者道：「兄弟，且莫悲切。這妖精把前門堵了，一定有個後門出入。你兩個只在此間，等我再去尋看。」行者道：「兄弟，且莫悲切。这妖精把前门堵了，一定有个后门出入。你两个只在此间，等我再去寻看。」
逐句解 Line by line
Pilgrim said, "Brother, no more of this grieving.
Nine characters, and Wukong has stopped mourning. Something has occurred to him between the blocked gate and the grave, and the change of tone is the first sign of it.
This fiend has blocked the front gate up, and there is bound to be a back way in and out.
The whole second half of the chapter turns on this sentence, and it is a deduction rather than a discovery — nobody has told him there is a back door. The cave's own name said so: the slab over the front gate read Folded Peak Interlinked Cave, and 連環, interlinked, is what chains of rings do.
You two stay here, and let me go and search again."
此間 is this place, a slightly formal turn. Wukong leaves the other two at the grave, which means the rescue that follows is entirely his own — the chapter's title calls him the Metal Lord and this is where he starts earning it.
Force has failed and Wukong switches to reconnaissance, which is the pattern of every problem he actually solves. The rod is for fighting; the thinking is what gets him past doors.
The clue was on the signboard the whole time. A cave named for interlinked rings has more than one opening, and nobody read it that way when Bajie was busy punching a hole in the door beneath it.