Vulnerable plot twist

I am writing a campaign with a rather vulnerable plot twist and am afraid my players will accidently bring the story to a short end.

In my preparation of the story I am writing the players get a mysterious story hook. Something they need in their town(s) get missing and they go out to the nearby city to seek advice.

An old librarian knows more about the legends in the area. But he needs a favour in return. He is an old man, he needs something from the mountain and could you please restore this ruin by pushing some rocks around while you're there. Easy.

Librarian turns out to be the man behind the mysteries in your towns and restoring the ruins was the last puzzle piece in his plan. Can you stop him before its too late?

The weak point in this story, in my opinion, is the part where the players get to decide if they want to help the old man. I'll have to make him very likable, persuasive and charismatic. What if they get suspicious. They could kill him, simply refuse or fool the man by telling they fixed the ruins and simply don't.

Help me out here, should I railroad this part, make some sort of incentive or hope my acting wil fool them in trusting the librarian?

Thanks in advance.

Edit:words