E1: From the start, you will arrive undetected at the southwest corner of the first building. Once you swim to the crates outside of the fence, Big Boss will contact you on the Transceiver. He will give you the mission instructions: infiltrate Outer Heaven, locate Gray Fox, and destroy Metal Gear. He will tell you to contact him on frequency 120.85. Then you will climb up and over the fence and be on your way. Head north to D1.
D1: Upon entering this room, you will be contacted again by Big Boss. Press F4 to listen to him, and he will remind you that this is an infiltration, and you should try not to be seen. From here, head north to C1.
C1: This room is guarded by two soldiers. Soldiers cannot see you if you do not stand in their direct line of site. With no weapons, you're only means of engaging them is by punching them until they are knocked out. Whether you choose to fight them or not, head west to C2.
C2: This is the lower left corner of the courtyard. There are three trucks parked here. The left truck contains rations. Because some items return when you leave a room and come back, you should visit this truck at least three times to max out your ration inventory. The middle truck contains a card and a soldier. If you enter the truck, the soldier will react and attack you. However, if you stand patiently outside to the left of the truck, the soldier will eventually exit the truck and patrol the right side. Then you can slip inside the truck and obtain access card 1 undetected. The right truck contains the binoculars. These allow you to preview the four screens on either side of you.
With card 1 in your possession, it's time to retrace your steps back to D1, and head east to D2.
D2: Two soldiers patrol around either tank. They both head up the middle, and split up at the top. As they head up, you can run to the center of the room. Then head up as the patrolling soldiers head down the opposite sides, and cross over the top to the right to reach D3.
D3: This room is split in half. You can only access the left half for now. The door at the top is locked with card 1. Press F3 to activate the card, and push up against the door to open it. Inside the room, a soldier patrols above a set of crate. He is tired, and tends to fall asleep. When he does, you can easily sneak up to him and knock him out with punches. Then approach the table in the back to collect the gas mask.
Having collected the gas mask, return back to C1, and proceed north to B1.
B1: Two soldiers patrol left and right in this room. The lower soldier won't be able to spot you as long as you walk between the crate and the left wall. You should time your progress through the room to remain unspotted, or to ambush the soldiers before they detect you. Head north to A1.
A1: Two soldiers stand on either side of the elevator doors, behind a row of tanks. The tanks will block their view of you while you stand below them. If you wait in the room long enough, their time to guard the elevator will end, and they will head out before the replacements arrive. That is your chance to run to the elevator. However, before you do that, visit A2 to the east first.
A2: Three more trucks sit inside the room. The doors in this room are locked with a card that you don't have yet, so all you can do is explore the trucks for now. Investigate the left truck to find your first fire-arm, a handgun. Though you now have a weapon, you still do not have ammo for it yet. The middle truck contains four soldiers who will not be pleased to see you if you enter, so avoid it. The right truck contains a soldier and a set of five land mines. You can try waiting outside for the soldier to leave, or you can just take the soldier on, avoid the bullets he fires at you, and punch him out. Collect the mines and leave the truck.
Once you have gathered the handgun and land mines, return west to A1, wait for the guards to leave their post, and enter the elevator. Your only choice will be to ride the elevator up to the third floor.
You should be returning to this floor after you have explored the third floor and gathered card 2.
A4: When you arrive back on this floor through the elevator, be careful not to get noticed by the guards that patrol either side of this room. Approach the right guard and punch him out, and then continue south to room B4.
B4: The three cameras that monitor this room make it impossible to stay hidden. The only sure fire way to guarantee your safety is to make use of the cardboard box that you collected from the third floor. Hop into the blind spot of the top right camera behind some crates and activate the box. Remain still as the left camera sweeps past your position. After it passes you on its way back down, activate card 2 and approach the door on the left (be careful not to get caught by the camera on the right). You'll find another prisoner to rescue inside. He'll inform you that Gray Fox is contained in an isolated cell. Exit the room and head to the south to room C4, using the box to avoid detection by the cameras if necessary.
If you've rescued every prisoner in the walkthrough so far, you will also be promoted to the second class. This will be indicated by the second star added below your health meter. Your health will be restored to its new maximum (around 66%) and you will now be able to store more ammo and rations.
C4: Two guards patrol either side of the passage. Punch them out or avoid them altogether, and approach the doors on either side. The left door is opened with card 1, and it contains a prisoner who will tell you that the best way to find the isolated cell is to get caught by the enemy. The right door can be opened with card 2. Inside, you will find the sub machine gun sitting on the desk. This is another weapon that is affected by the silencer, but you'll need to find more ammo before you can use it since it uses different bullets from the handgun. Explore both rooms, and then head south to room D4.
D4: Two guards patrol this room. One stays near the tanks at the top, while the other circles around crates. Sneak up on them and punch them out, and then approach room D3 to the west.
D3: You may recognize this room as the other side of the location where you found the gas mask. You won't get to walk very far into this room when suddenly a guard sneaks up behind you and orders you to freeze. Another guard will assist him and the two will arrest you. The next thing you know, you will find yourself in an isolated cell…
You should be returning to this floor after leaping from the roof with the parachute.
B3: Fortunately, you will arrive safely in the center of the courtyard if you had the parachute. Unfortunately, you will land in the middle of a pack of sleeping dogs. Prepare your handgun and remove the threat before they can hurt you. You will notice that none of the access cards you have will open the door above you. Nor will it open the doors in B2 to the west. Head south to C3 first.
C3: You will see two trucks here. If you walk carelessly across the ground, you will likely trigger many explosions that will do a good deal of damage to Snake and possibly kill him. To avoid this, activate the land mine detector. When you do, you will see a collection of red dots on the floor. These dots indicate the presence of a deadly land mine that you should avoid. Approach the entrance of the right truck but do not enter the left truck first. If you do, you will be taken back to C2, and be forced to jump from the roof again. Enter the right truck and you will find access card 4 waiting for you. Now if you happen to enter the left truck and get transported to C2, it won't be so bad, since you can simply re-enter the courtyard through the southern door of A2 with card 4. From the truck, return north to B3.
B3: Take out the dogs and approach the door with card 4 activated… but to your dismay, you will discover that Dr. Pettrovich has already been moved from this cell. Exit the room, and head west to room B2.
B2: Watch out for the two dogs napping here, and approach the left door with card 4. Inside is a prisoner who will tell you that Dr. Pettrovich has been moved to building 2 when you rescue him. He will also tell you that building 2 is directly north of building 1. Armed with this new information, it's time to make your way out of this building and on to the next.
At this time, your immediate objective is to open the north door in room A3. To get there, you can open the right door in B2 to reach A2 (where it is a very good idea to stock up on land mines), and open the door on the east wall of A2 to reach A3. From there, open the north door and you will be on the path to the second building. However, if you take a little time to explore the basement, you may find a valuable item to collect by using card 4 before you leave.