Museum Piece

Museum Piece is a mission given to Niko Bellic by Ray Boccino in Grand Theft Auto IV. In this mission, 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 Brotherhood.

This mission is a famous mission as it rewards Niko the Impossible Trinity trophy, sets in motion all of Ray's future missions, and being a part of the Trinity, is the only mission in the whole GTA IV Era where Niko Bellic, Luis Lopez & Johnny Klebitz (the three protagonists) appear at once.

Overview
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 The Libertonian museum in midtown Algonquin to meet up with Johnny Klebitz, a member of The Lost Brotherhood 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, a gunman ambushes the trade and kills Mori. Isaac takes the diamonds while Johnny snatches the money 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.

Walkthrough
After the initial cutscene, head over to the Libertonian museum in Middle Park, Algonquin. Meet Johnny 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.

Remain in cover and take out any Mafia or Jewish Mob members. Move forward down the lane and regain cover when Niko begins to lose health (or armor). Return fire to everyone that is in sight. Once the player is at the end of the lane, turn left and left again to go down the stairs. Shoot the enemies coming up the stairs and get in cover near the bottom of the stairs.

Kill the rest of the Mafia and Jewish mobsters (take your time) and go out of the door (South of the North Entrance). Follow the path until you get out of the museum. Eliminate anyone in the way. Once Niko gets outside, shoot down the drivers of the cars parked outside and lose your wanted level. Isaac's Crew will give chase in their cars, attempting to kill you, once you have lost them you will call Ray and explain what happened. (The achievement/trophy Impossible Trinity is gained on completion)

Tips

 * Make sure to get the armor pickup once Niko gets to the bottom of the stairs. It's hard to miss, but go left at the bottom of the stairs and find the armor in a small, low booth.
 * An SMG would be useful in this situation as it can quickly neutralize some of the goons. Thus, it is revealed that Niko uses one during the mission from both Johnny and Luis's point of view.
 * A good strategy is to use Dwayne's special ability and get 2 of his goons to back you up.

Trivia

 * Johnny is seen running out of the museum with no weapon.
 * There is no ladder where Issac hides, in TBOGT Luis escapes to the roof by climbing a ladder
 * It is possible to kill Johnny at the side entrance because you can see him waiting for Niko and when he is escaping (if Niko quickly follows hims).But he will fail the mission if either is done, and Ray will mention that the Lost MC is going to have a big problem with this.
 * This mission is seen again as Collector's Item in The Lost and Damned, this time from Johnny's point of view and again as Not So Fast in The Ballad of Gay Tony, this time from Luis' point of view.
 * The Desert Eagle is first introduced without cheating (unless you killed Playboy X and took his in The Holland Play, which may or may not have been played before), 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 Issac 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 Issac and takes the diamonds right from his hand. If Issac is killed, the mission will fail (text displayed will say "Issac is dead.").
 * 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.
 * Outside the North entrance, a Hellfury can be seen, as Johnny's getaway vehicle. This is because the Hexer had not been included in the final version of GTA IV.
 * If you are quick enough and follow Johnny out his exit, he becomes a random pedestrian and walks around instead of escaping. He also has unlimited health even if attacked by the Jewish Mob. Only the player can kill him.The mission will not fail once Johnny reaches the North Entrance and stops.The best way to follow Johnny is to climb over the railing and fall to the floor below.
 * If you go back to where the deal took place, you will see three corpses, however, instead of Mori Green, there will just be an old fat Jewish man in his place.
 * After escaping the museum, the instructions state to "Lose Issac's Crew", although if you kill the pursuers and look at their bodies, they are actually Ancelotti goons.
 * It is revealed in The Lost and Damned and The Ballad of Gay Tony in both Johnny and Luis' point of view that Niko used a SMG in this mission.
 * Most of the mobsters are replaced with policemen in The Ballad of Gay Tony, which leads to a continuity error.
 * This mission, Collector's Item, and Not So Fast are the only missions where all three protagonists appear at once.

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