To begin this quest, look for Isaac Karlin in Biosciences. He would like you to deliver some genetically modified seeds to the owner of Warwick Homestead, Roger Warwick. Be sure to listen to Isaac's instructions, especially since this mission involves speaking a pass phrase to Roger.
Roger has been replaced with a Synth version of himself. Even though he happens to be a better husband and father than the original version, this secret cannot be revealed, especially to his family. It's your job to silence the locals who suspect Roger's true identity. To do this, you need to engage in friendly conversation with Roger's family to get a name. In our playthrough, it was Roger's daughter who had the key name: Cedric Hopton.
You'll find Cedric in Goodneighbor, in a bar called Third Rail. His information comes at a price: 500 caps, which hopeful shouldn't be a huge expense this far into the game. You'll learn that a settler named Bill Sutton is suspects that Roger is a Synth.
When you return to the homestead, you'll find Bill in a standoff against Roger. If you can't use your charm to convince Bill to lower his gun, you'll have to kill him. Whatever the result, you can then complete the quest by speaking to Isaac Karlin again.