Walk away. You'll have other ideas at least as good as the one you're looking at now, and a dispute over ownership is no way to start a successful company.

People will rush to tell you that ideas aren't worth anything and simply having an idea gives your erstwhile prospective cofounders little claim on it. That's true, but not dispositive. If you start your own venture based on an idea they believe they entrusted to you, there will be drama. You have no guarantee that the drama won't take a form that requires lawyers. Either way: it'll be ugly.

Life is too short. Do something else.