Twin Suns refers to Solid Snake's mission to stop Liquid Ocelot from acquiring Metal Gear REX's rail gun, on the abandoned Shadow Moses Island.
"Return to Shadow Moses"
Through the Metal Gear Mk. II, Solid Snake and Otacon were able to hear everything said on Liquid Ocelot's boat, during its escape on the Volta River in Eastern Europe. Although the Mk. II was discovered and destroyed by Vamp, they were able to hear enough to figure out Liquid's true plan: to take down the Patriots himself by destroying the main satellite of JD, the core AI. To do that, he needed a weapon that was powerful enough to fire a missile into space, but was not governed by the Patriots AI network, as Liquid could not access WMDs due to it being governed by JD, and not Sons of the Patriots.
As Snake and Campbell wondered how Liquid would be able to achieve this, Otacon deduced that Liquid was planning to use Metal Gear REX, its rail gun being the only WMD left outside the Patriots' control to be able to launch a nuke. Deducing that Liquid was planning to revisit Shadow Moses (and confirmed by Mei Ling), Snake followed him to Alaska, and to the one place he never thought he would return, re-navigating the route he took almost a decade earlier to reach Metal Gear REX's Underground Base. Raiden, despite Sunny's protests of his dialysis not being complete yet, also attempted to tag along with Snake, largely because he believes that only in aiding Snake take down Liquid and the Patriots can he truly be free from being a pawn to the proxy. However, Snake refused, realizing that Raiden misinterpreted his advice years ago, and also informed him that he "cannot see the day" as long as he follows him, and eventually admitting that Raiden at least has a family to go back to, while Snake himself has less than a few months before he becomes a bioweapon. Raiden, however, snaps at Snake and reminds him that he doesn't possess a family (referring to his child's miscarriage and Rose's separation from him and her remarriage to Campbell) and that he, like Snake also has nothing, before collapsing. Before losing consciousness, also begged Snake to "not leave me alone." While on the way to Shadow Moses, Solid Snake had a dream about his first visit to Shadow Moses, one which he abruptly wakes up from in a panic.
Mei Ling displays a satellite image of Shadow Moses in 2014, with Outer Haven shown docked at the northeast coast.
"Cold Memories"
Otacon inserted Snake at a snowfield west of the heliport. After evading a patrolling Gekko, Snake once again arrived at the helipad.
As he made his way to the nuclear warhead storage building, Snake was visited by a series of haunting flashbacks while evading the Dwarf Gekko. Otacon suggested this time that Snake can enter the snowfield through the back of the Storage Building rather than going through the commander's office in the basement. Snake activated the door by retracing his steps to Otacon's old office with the new Metal Gear Mk. III, but not before witnessing Naomi Hunter and Vamp via surveillance camera on their way to REX's hangar.
"Crying Wolf" and "Vamp"
- "Cry. Until your eyes run RED!"
- ―Crying Wolf
Crying Wolf
Snake emerged into the snowfield where he fought and killed Sniper Wolf, encountering, to his shock, her doppelganger, BB member Crying Wolf with a rail gun. After engaging in a lengthy sniper duel, Snake defeated her and continued to the Underground Base. However, Snake had to make a detour from the blast furnace to the casting facility after the elevator he used nine years ago to get to the warehouse was locked. Once there, Otacon used the Metal Gear Mk. III to reactivate the elevator, taking them up to where REX was still laying unmoved, exactly where Snake had left it. Unfortunately, also waiting for him were Vamp and Naomi. Snake fought Vamp, and as he was already well aware of Vamp's regenerative capabilities, managed to subdue him with a CQC administered nanomachine-suppressor injection.
"Atonement"
Vamp was now mortal, but still just as deadly. Suddenly, Raiden, fully recovered after a white blood infusion, appeared to fight Vamp while Snake fended off the Suicide Gekko swarming the base. Snake destroyed all the Gekko, and after receiving many wounds, Raiden managed to pull off a devastating swipe to Vamp very much in the way he had killed Solidus. Vamp, now having mortal wounds for the first time, was euthanized by Naomi by a full dose of nano-suppressant.
With Vamp defeated, Naomi apologized to Snake and Otacon, before revealing that she had cancer, and, overwhelmed by guilt, injected herself with the syringe. Having effectively committed suicide, Naomi died in the same place her adoptive brother, Gray Fox, had. They mourned for their loss, accepting that they would never know where Naomi's true allegiance had lied.
"Fight of the Metal Gears"
- "Moses! Where our fates were born. And where yours ends, Snake!"
- ―Liquid Ocelot, on board Metal Gear RAY, prior to fighting Snake's REX
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Solid Snake at the helm of REX.
With more Suicide Gekko on the way, Otacon used the Mk. III to reactivate REX. With Raiden hanging on, Snake used it to escape from the collapsing base. Upon emerging, Raiden was crushed by falling rubble, and Snake was confronted by Metal Gear RAY, being piloted by Liquid. Though REX was handicapped due to the loss of its rail gun and radome, it still had much of its previous weaponry with it, such as the anti-tank missiles, free-electron laser, and machine cannons. Otacon also activated a program, secretly installed on board REX while it was still in development, which utilized its hull as a weapon in itself, drastically increasing its chances against RAY, although it could only be activated in certain situations. Though RAY had been specifically designed to destroy REX and its many variants, Snake was still able to defeat the more advanced Metal Gear model.
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RAY, piloted by Liquid Ocelot.
Followings RAY's defeat, the machine crashed to the ground, with Liquid Ocelot stumbling from the cockpit. Lying on the ground, Liquid stretched out his arm and called to Snake, apparently in some pain. Reminded of events from nine years earlier, Snake speculated as to the cause of Liquid's suffering and exclaimed the word "Fox...," after which Liquid finished with "DIE!!!," both partaking in an act of role reversal. Liquid then collapsed, but immediately raised himself from the ground, having only been toying with Snake, and claiming that the FOXDIE virus would not work this time. Liquid then made his escape towards the pier, laughing as Snake attempted to give chase, the latter's arm and leg having been injured by REX's rough landing during the battle's aftermath.
"Everything Ends"
Outer Haven surfaces at the Shadow Moses port.
Snake struggled to catch up with Liquid, until he noticed a figure emerging from the ocean moments later. The form was that of Liquid's Outer Haven, an Arsenal Gear model warship that had been stolen from the Patriots. As Liquid boarded the ship, he boasted that he would destroy JD with REX's rail gun, and bring the Patriots' System under his control via GW, at which point everything would end, and then begin anew. Snake fired his rifle at Liquid, but his injured arm prevented him from aiming accurately, and he fell to his knees in exhaustion. Liquid then directed Haven to turn about and then ram into the pier, hoping to kill the weakened Snake. Meanwhile, Raiden, having cut off his own arm to break free of the collapsed rubble, raced to Snake's aid, using his cybernetically-enhanced strength to stand in the ship's path, and prevent Snake from being crushed. Similar to Gray Fox before him, Raiden yelled for Snake to get away as the machine's weight pressed down on him. With the pier crumbling away behind Raiden, Snake finally came to and dragged himself away, just as Raiden's strength yielded to Haven. The Cyborg Ninja was then seemingly crushed beneath the ship, yelling out the name "Rose" as he fell.
Outer Haven eventually retreated when the USS Missouri, now captained by Mei Ling, arrived at the port and opened fire on the ship. When Snake looked down at Raiden's body, the result of Raiden's actions was enough to send him into a coughing and wheezing fit out of disgust. Snake and Raiden were later recovered and carried aboard the Missouri.
Aftermath
Raiden would ultimately survive the near-death experience, and would hold off the FROGs blocking Snake's way to the microwave hallway. Likewise, Snake and Otacon ended up uploading FOXALIVE and halting the rail gun launch, although Liquid later revealed that FOXALIVE was in fact his real plan. After the mission, Raiden would be reoutfitted with a more human-looking cyborg body, but would ultimately join a PMC in large part because he couldn't get a civilian job because even the human body looked unnatural. When he lost his left arm to a duel with Samuel Rodrigues in Africa, he angrily lamented the loss of his left arm again, referring back to the events of this mission.
Behind the scenes
"Twin Suns" is the title of Act 4 in Metal Gear Solid 4: Guns of the Patriots. It is a reference to Solid Snake and Liquid Snake, as well as the remake of the original Metal Gear Solid, The Twin Snakes. The whole act is one of nostalgia as it is a reference to the debut of the rivalry between Snake and Liquid.
The player controls Metal Gear REX in the fight against Liquid's RAY.
According to an Integral Podcast for Metal Gear Solid 4, Act 4 was originally intended to be more action based, with their planning an alternate route to the canyon from the starting point where they could find a frozen lake with patrolling Gekko, and the player would have the option of shattering the ice below the Gekko to have them sink to their watery graves, as well as a secret base below the Warhead Storage Building, but these were cut when it was decided to make Act 4 more nostalgic instead.
In the Act 3 Briefing, alongside that of the briefings for Acts 1, 2 and 4, if the player uses the Metal Gear Mk. II (and in the case of Act 4, Mk. III) to interact with the area up in the kitchen area of the ship, they can interact with Sunny's clipboard. The clipboard, depicting the egg schedule for the Nomad's chickens, showed that the month of March had less eggs than January or February, yet April did not have any tallies on egg productions. Whether that was meant to imply that the events of the missions occurred in March is not made clear. In addition, when Liquid Ocelot is shown in the monitor in Act 1, the date for the last login is instead shown to be Friday, July 4.
Gameplay
If the player dies during this act, flashbacks of the events of Metal Gear Solid appear onscreen, and the original Game Over music will play.
If the player manages to switch the camera to overhead view during the act, Solid Snake will utter "Overhead View - Just like old times.", referring to the original game's usage of the overhead view. Doing so after downloading a trophy patch will result in a trophy achievement of the same name being unlocked.
After defeating Crying Wolf, if the player shoots one of the wolves that appear, Otacon will become disgusted, and the other wolves will attack them and will keep attacking either until the player leaves the area or until they're all shot.
After completing the game once, when the player has to distract the Gekko while the Mk. III is unlocking the door, the player can use the Crying Wolf's rail gun to kill the Gekko instantly. Also, after the Gekko is killed, the player won't have to wait until the timer expires, instead, the player only has to wait about 5 seconds after the Gekko explodes.