By Reason, by Reflection, by Everything: A Pride and Prejudice Variation

❝The promise, therefore, was given and must be performed.❞ - Jane Austen

What if Mr. Thomas Bennet's first-born daughter is promised to the elder Mr. Darcy's first-born son?
Are promises made always promises kept?
Or is a love like Fitzwilliam Darcy and Miss Elizabeth Bennet's destined to prevail?