Ray wants Niko to sell the load of diamonds they obtained in the earlier mission to a buyer at a deal in a museum. Johnny Klebitz will be there as well with Niko to provide support and take his cut of $1 million back to The Lost MC. However, the deal goes horribly wrong upon an interruption.
Ray meets Niko at Drusilla's and they talk in the kitchen after Ray hangs up his phone. Ray yells at Niko for not calling him sooner since he needs him to sell the diamonds. Niko tells Ray that he will only do it if he promises to find Florian Cravic for him, to which Ray reluctantly agrees. He then sends Niko to The Libertonian museum in midtown Algonquin to meet up with Johnny Klebitz, a member of The Lost MC who had already participated in another deal with Niko, so they can sell the diamonds to the Jewish Mob representatives, Isaac Roth and Mori Green. During the deal, Luis Fernando Lopez, a guy who is a buddy of Tony Prince, ambushes the trade and kills Mori. Isaac takes the diamonds while Johnny grabs the money, Luis tries to take back the diamonds from Isaac, and Niko is forced to shoot his way through Isaac's henchmen to exit the museum empty-handed. After getting out, killing Isaac's men that were lying in wait outside and escaping police pursuers, Niko calls Ray and informs him of the what went on in the museum. A furious Ray tells Niko to meet him later so they can deal with Johnny.
In order to complete the mission the player must:
- Meet Johnny at the side entrance to the Libertonian.
- Escape the Libertonian.
- Lose Isaac's crew.
After the initial cutscene, Niko heads over to the Libertonian museum in Middle Park, Algonquin. He meets Johnny Klebitz at the side entrance to the Libertonian. They meet the buyer, Isaac Roth with his men inside. A cutscene shows the diamond deal ambushed by Luis Fernando Lopez. Johnny takes the money and flees, and Luis steals the diamonds from Isaac, while Niko has to shoot his way out of the museum.
Niko remains in cover and takes out any Mafia or Jewish Mob members. He then fights his way through the enemies and when he is at the end of the lane, he turns left and left again to go down the stairs. He shoots the enemies coming up the stairs and gets in cover near the bottom of the stairs. A armor pickup can be obtained once Niko gets to the bottom of the stairs in a small, low booth.
He kills the rest of the Mafia and Jewish mobsters and goes out of the door (South of the North Entrance). He follows the path and gets out of the museum, eliminating anyone who tries to block in the way. Once the player gets outside, they can immediately shoot down the drivers of the cars parked outside and must lose the potential wanted level. Isaac's Crew will give chase in their cars, attempting to kill Niko, Niko will call Ray and explain what happened once losing the enemies. (The achievement/trophy Impossible Trinity is gained on completion.)
- Mori Green - Killed by Luis in order to retrieve the diamonds.
- Isaac's crew - Killed by Niko in order to escape.
- There is no ladder where Isaac hides; in TBoGT, Luis escapes to the roof by climbing a ladder.
- The Desert Eagle is first introduced without cheating, because many Jewish Mobsters apparently hold them. This is because the Desert Eagle is an Israeli weapon.
- If the player wanders back to where the deal took place (during the mission), they can find Isaac Roth hiding behind some crates bad-mouthing Ray Boccino, and mourning the loss of Mori Green, though in Not So Fast, Luis Lopez knocks out Isaac and takes the diamonds right from his hand.
- Before descending to the bottom floor, the player can shoot at the hanging dinosaur bones, if done properly, the bones will fall onto unsuspecting mobsters below.
- After successful completion of this mission, the Xbox 360 Achievement/PS3 Trophy Impossible Trinity is unlocked. In The Ballad of Gay Tony, completing the mission will unlock the Diamonds Forever achievement.
- The achievement Impossible Trinity refers to the three protagonists of the GTA IV Era coming together in one place at the same time.
- During this mission and Collector's Item, Luis is not seen during gameplay. He would not be seen anyway as he snatches the diamonds and escapes upwards, before Niko and Johnny are out.
- After escaping the museum, the instructions will state the player to "Lose Isaac's Crew", though they are actually Mafia goons.
- Most of the mobsters are replaced with policemen in The Ballad of Gay Tony, which leads to a continuity error.
- The mission can be completed by taking the subway. If the player runs on a straight line after exiting the Libertonian (having killed the thugs guarding the exit), he will find the subway entrance to the Middle Park Station. Isaac's crew will only follow Niko down there if he takes too long to catch a train (which most likely will come just as he arrives at the platform). Since the subway follows its own path instead of that of the streets above, it can be seen as a shortcut, and the mission will be completed before the train arrives at the next station.
- This is the last time Niko and Johnny have crossed paths, and the second time Niko and Luis have crossed paths.
- If players walk to where Luis Lopez escapes, the window is open, and he may say something about Mori. He cannot be seen though.
- The cop who will appear soon after the shootout will be armed with a Combat Shotgun instead of a regular Shotgun. This also happens during The Snow Storm.
- Niko can steal Johnny's Hellfury and park it at a safehouse without failing the mission.
- After completing the mission, Niko can call Roman to talk about how he is working with the Mafia and the Lost MC on the diamond deal and how “this Dominican looking guy shot the place up“.