This is the third quest you must complete to reach Jing Lang and obtain the Templar Key in her possession. You can start it after completing the "The Other Brother" quest. To do so, head to the tavern in Nassau and talk to Upton, whom you will find there.
Edward ushers Upton to safety in order to protect him from his brother, Vance.
Right after the cutscene ends, you will have to defeat three drunkards in a fistfight in the tavern who attack you.
Follow the slow-moving, drunk Upton. When he stops near the dancers, pay them 100 Reales to leave you alone, allowing Upton to move on.
When you walk a bit further, Upton will stop by one of the houses to relieve himself. This will displease the approaching guards, who attack you after a brief argument with Upton.
There will be three guards. You do not have to wait for them to attack first - simply approach them, perform a double assassination on two of them, and then finish off the last one.
Once the guards are dead, keep following Upton. He will walk a bit further before stopping near one of the nearby houses.
Talk to Upton. Once you finish, Upton will move to a safe place and this quest will end, allowing you to start the next one - "Queen of Pirates, King of Fools".
