This is the first quest in the Upton Travers series, which will bring you closer to acquiring one of the Templar Keys. To start it, head to Nassau after completing the "This Old Cove" quest and talk to Vance Travers.
Edward meets Vance Travers to inform him of the growing Templar threat.
Hint:
Head to the market in Nassau - it is best to move along the rooftops so that you are not spotted by Upton's men when you arrive, otherwise you will have to fight.
Hint:
The two men will start moving. Follow them without being spotted, ideally by staying on the rooftops. After a while, they will turn behind one of the buildings.
Move to the back of the building where the men turned. Upton - the man you are looking for and whom Vance ordered you to kill - is waiting for them in the courtyard behind that building.
Take Upton down in any way you choose - for example, with a well-aimed headshot.
If you killed him from a distance, you still need to approach him. Do not worry about other enemies - as soon as you get close, a cutscene will play. It turns out that Upton is the true bureau leader, and Vance - who is his brother - lied to you.
This quest will end, and the next quest in the series will be unlocked - The Other Brother.