In your new contact page, you will have a Account model - which is populated by a URL paramater that will come from the prior page. The Account model can include parent data (information about the Plant). When you are choosing fields for your model, any parent reference field will have a link icon next to the selection checkbox. It will take you to the fields of the parent object which you can select to include in your model.
Then on your NewContact model you will create conditions using the “field from another model” value category and do the same sort of traversal to get to the selected “grandparent” field. In the field picker popup - the same icon is avialable to get you to parent data. 
