the (thing, one) you must see!

[keannu]

When you say "the one", does "the one" have to refer to a word appearing ahead of a sentence? I explained to someone the answer should be "the one" as it refers to "musical" while "the thing" doesn't as it doesn't refer to anything. But I suspect I explained wrong. Why doesn't "the thing" work and why does it have to be "the one"? It's probably "the one" is something unique regardless of referring to something.

2p,q)No musical has been more attractive than Jekyll & Hyde. This is the greatest musical, and the (thing, one) you must see!​

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[Raymott]

When you say "the one", does "the one" have to refer to a word appearing ahead of a sentence? I explained to someone the answer should be "the one" as it refers to "musical" while "the thing" doesn't as it doesn't refer to anything. But I suspect I explained wrong. Why doesn't "the thing" work and why does it have to be "the one"? It's probably "the one" is something unique regardless of referring to something.

2p,q)No musical has been more attractive than Jekyll & Hyde. This is the greatest musical, and the (thing, one) you must see!​

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It has to be "one". Yes you explained it correctly. By saying"one", you are saying it's the one musical that you must see (if you could only see one).
You couldn't use "one" in this sentence: "This is the greatest musical, but the thing you must see is the circus across town."