For a non-spoilery description of the Crystal Cavern Conundrum quest, go here.

Spoiler warning!
This article contains game/plot details regarding an area or quest .

If you get stuck, leave the area by going to town, your farm, or an event. When you return, you will be on the left bank next to Inspector Kaine. Talk to him to reset the puzzle.

Once the puzzle is solved, Inspector Kaine will join you on the far side of the plank puzzle area. Be sure to collect your reward before leaving the area! It has been reported that if you leave without collecting the reward, you can't get back to the right bank, so you can't collect the reward.

Plank Puzzle Solution:

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All credit for photos goes to Andrea, who posted them on the Codename Entertainment forum.

I made this a separate page because I couldn't figure out how to put photo sliders inside a spoiler block. I hope someone with more wiki editing experience will incorporate this into the main Detective Agency page. Also, if anyone knows how to make a slideshow paused by default, that would be an improvement.