Story related. No need for a guide if you don't like spoilers.
*** Spoiler - click to reveal ***
You will start this episode with a flashback of Javier talking to Kate while she is washing the dishes. She accidentally destroys David's glass which he got from his captain in the army. You can either choose to let Kate and David fight it out or keep yourself in the background.
Afterwards, you'll be back in the present and you need to treat Kate's wound with the doctor. You'll need to extract the bullet before she can stitch it. Either allow Gabe to stay or send him off. After Kate's operated successfully, head outside the gate to Gabe and have a chat with him.
Either give him Ariana's walkman or keep that to yourself. After a while you'll notice guys attacking you. Run back to Prescott and the achievement should unlock.