Far Cry 3 Piece of the Past follows the Saving Oliver main story quest. You have saved Oliver but there is one more friend you need to save, Keith Riley.
In order to save him you will need to work with Buck, hit-man that works for Hoyt.
Exit the cave.
After your chat with Liza you will need to make your way out of the cave. Upon leaving it you will receive a phone call from Willis Huntley. He informs you that a man named Buck, a hit-man for Hoyt, purchased your friend Keith Riley.
Go talk to Buck.
Buck can be found in Badtown. Fast travel there and an Objective point will lead you to Buck in local bar. Buck tells you that he has your friend Riley and that he will return him to you, after you acquire something for him. What he wants is nothing less than the ancient knife you saw in your visions earlier in the story. Buck makes it clear that getting him the knife is the only way you will be getting Keith back. You will need to head to the Nostalgia, a boat that should hole your first clue to the location of the knife.
Infiltrate the Nostalgia.
The Nostalgia can be found far to the east. You can use a jet ski to get there, but be aware the guards on the Nostalgia can spot you if you get too close. When you reach the boat you can climb up one of two ladders you can find at the sides of the boat. One is at its rear side and the other one at its left.
Find the entrance.
There are couple of pirates you need to defeat here, the toughest one is the one that is guarding the door you need to pass through. This heavy gunner can be defeated with bombs, lining up some head shots, or with couple of close range knife hits.
Infiltrate the Nostalgia.
When you get inside of boat you will find a couple of pirates you need to defeat and an alarm you need to shut down. You will come to a couple of doors you need to pass through until you reach a big door that leads you to room you are here for.
Hack the laptop to find the knife.
Be on alert since this room is guarded by a heavy pirate. The laptop that you need to hack in order to find the knife’s location can be found on a nearby table. You will also pick up the nearby compass. After you are done with downloading the information an alarm on laptop goes off. You discover a bomb under the table and sprint out of the room. The bomb explodes and the boat start sinking.
Escape from the Nostalgia.
Water is rushing into the boat, and some of the rooms you just came through by will be underwater. You need to swim through them in order to get to the parts that are above water. Use the appropriate button to swim faster, and the oxygen cans to take additional breaths while you are underwater, in order to make it through the ship alive . After you pass the first closed door a pirate will try to stop you. Here you need to respond fast and use the appropriate buttons as fast as you are able to. A little further on you will find on a glass window that will let you leave the ship. Soon after that you will be found on shore by a Buck. You inform him that you found a compass and he tells you to follow the trail.