This segment starts off with you controlling Snake in MGS1. Avoid the guards and head to the top-left of the screen and crawl into the vent. This segment doesn't seem to reflect on your gameplay, so don't worry about specifics. Just get to the end. If you die, it's not actually a game over - just hit "exit" and Snake will snap out of it. Either way, you'll continue with your real mission.

Once on the ground, scour the area for weapons and then head west. There's a Gekko hidden in the snow - use your Solid Eye to watch it and avoid it. Move slowly and deliberately. Use the tunnel on the northern side of the area to collect some goods, and remember to stay as camouflaged as possible. You want to head west, so do so when it's not looking. Stay on your belly whenever possible. When it's definitely facing away from you, get up to a crouch-walk and make a break for it. Now you're at the Heliport from MGS1. This entire area is full of flashbacks, so keep an eye out. Collect the Chaff grenade from the Helipad - there's only a few in the entire game, and they're really useful in this part. Check the storerooms on the west side for rations and ammo - the middle room can only be reached by going through the tunnel in the southern room. There's a Mk. 23 pistol under the truck in the north. Go near the elevator for a ration and a flashback that's worth 1000 Drebin Points. When you're done, don't walk in the front door.

There are two vents - one on the first floor on the left, and one on the second floor (the railing over the gate) in the middle. I chose the latter. Follow this vent all the way around and you'll get an iPod song. This area is full of Scarabs - those robotic multi-armed things you encountered earlier. Drop a chaff grenade and drop down. Once it wears off, use nightvision to spot the lasers the scarabs use more easily and stay out of their path. Crawl low and use OctoCamo (with the facemask). Move under the door to the north. Stay out of the path of the lasers and crawl through. The lines in the floor mark what they'll trace, so use that to stay safe between movements. Once you're past that, you'll be in the Canyon. Move ahead and there will be two Gekkos camouflaged in the snow. Crawl across the snowfield and they'll never even wake up (it's sound that does it, so if you ran ahead they'd wake up). To the left of the door you need to go through is an iPod song.

Now you're in the nuclear warhead storage facility. There's a ration to the left here. Move forward and crawl under the door. After the codec conversation, check the back of the truck ahead of you for a Stinger missile. Go forward to the panel near the door. You can't unlock it. The code you'll need later is 48273. Go to the left and take the elevator down to B2. Once down there, go to the right to get a LAW rocket. Keep checking the left side to get an iPod track (part 2 of the 20 year history of Metal Gear Solid). Head to the right side of the map and walk down the hallway. Head north into Otacon's office - after a short cutscene, he'll ask for the code. Inputting the code from earlier gets the power back on; forgetting it gets it up anyways, but Snake takes some psyche damage. Input Drebin's frequency - 148.93 - for 100,000 Drebin Points (it's still a wrong answer technically, but you'll hear his monkey laugh if you do it right). The power comes back on. Head out and a Gekko will block the hallway. Back up before it spots you.It'll move through the hallway. When it's looking toward the elevator, roll to the left side of the map and craw under the desks. Move up to the generator unit.

Take out the Metal Gear Mk. III and press triangle to electrify the floor it's standing on. Call the elevator and head back upstairs. Go over to the door from before and the Mk. III will start unlocking on it. A Gekko will come out of the elevator - you need to distract it so it doesn't spot it. This Gekko is invincible, so you just need to distract it until the timer is done. Throw magazines and smoke grenades to lure it away from the Mk. III, but be aware that it moves fairly viciously - for example, when jumping down from the catwalk, it will easily crush you under its weight. Stay camouflaged, and try not to start anything up with it or it'll get antsy. If it spots the Mk. III, that counts as an alert. Use silenced weapons and aim for its legs if it's pressuring you. If you get desperate, a chaff grenade will buy you some time. All in all, the best strategy is to stand in front of the truck and throw smoke grenades to the back of the room. Keep this up and you'll have bought enough time for the door to open. As soon as it is, run out of there.

Metal Gear Solid 4 Walkthrough