The story flashes back 20 years, when a 15-year old Nathan Drake arrives at the Museo Marítimo located in Cartagena, Colombia and enters the museum looking for Francis Drake's ring. He enters an exhibition called El Pirata Francis Drake, “The Pirate Francis Drake”. Nate eventually finds the ring in a display case on the 2nd floor.
Nate notes down information and sketches the object in his journal, when a middle-age Victor Sullivan walks in. Nate hides beside a display case and watches Sully insert a key into the display case's lock. As closing time approaches, museum guards grab Nate and throw him into the street. Nate spots Sully outside the front entrance, and pursues him along the streets of Cartagena, where the player learns how to hide and use cover. Sully enters a building, and Nate climbs up to peek through the window.
Sully gets another cabinet key cut and places it in his wallet. Sully leaves, and Nate follows, seeing Sully meet up with Katherine Marlowe. As the couple stop at a merchant's booth, Nate pickpockets Sully for his wallet. Nate walks away slowly, until Sully appears, asking for his wallet back. The two engage in a conversation, with Nate first denying he has the wallet. He compliments Nate on such a nice lift, saying he is pretty good. Nate continues to deny it. Sully then claims that Nate's technique is pretty sloppy as he has been telegraphing all of his moves. He then insists on calling the police unless he hands him his wallet, but Nate doesn't mind, as he believes he doesn't like the cops anymore than he does. Nate hands it back and run off, with the key.
I. Nate's journal (tap 'Select' to open it) contains a picture of the display case that he is seeking, though there's no immediate hurry to reach it. Feel free to study the other exhibits (press up on the D-pad when the prompt appears) and be sure to collect treasures #3 and #4. You can actually unlock a trophy by interacting with almost every exhibit in the museum before you approach your goal. Examining the appropriate display case in the upstairs gallery triggers a cutscene.
II. Your objective outside the museum is to tail Sully without being detected. You can achieve this by using the cover system (follow the prompts as they appear), and by maintaining a safe distance when no suitable hiding spots are available. Collect treasure #5 directly outside the museum, then grab treasures #6 and #7 before you follow Sully into the meat market. Approach the door and press triangle to pass through it.
III. Climb the drainpipe and Nate will automatically transition to swinging when you reach the horizontal bars; on the final bar, hold L towards the nearby ledge and tap X to jump when you have sufficient momentum. From here, traverse to the right to arrive on a lintel beside the second window along to see what Sully is up to.
IV. After the short cutscene ends, climb to the right. When the pipe breaks, swing over to the balcony, then run to the far side to track Sully's movements. Collect treasure #8 from beside the potted plant, then jump over to hang from the other side of the balustrade. From this position, tap X while hanging from the sign to transition to the opposite side. On the rooftop, jump onto the Hotel Arrecife sign and collect treasure #9 at the top. Press O at slow, regular intervals to safely return to street level. On the ground, head up the nearby steps, then follow Sully and his female companion across the square.