Half-Life Decay is a DLC to the popular game Half-Life. It is a first person shooter originally released for PlayStation 2 on November 14, 2001 but was later ported to PC as downloadable content in November of 2008. Half-Life Decay is the only game in the Half-Life series that was designed to be played cooperatively by two players. However, the game can also be played single player but will require frequent switching between the two main characters, Dr. Gina Cross and Dr. Colette Green, to complete the missions.

The game’s setting is the same as the original Half-Life game. Players will be battling through missions in the Black Mesa Research Facility. There are nine missions in total, each full of tricky puzzles and tough enemies. Players will have to think clearly where to place the two main characters during the puzzles and the fights, as bad placement can result in the failure of a mission! Some special notes about Half-Life Decay are that players cannot save in the middle of the mission, meaning one must play through the entire mission successfully to unlock subsequent missions, and that the non-active character (single player) will fire at enemies automatically but only at the general direction they are facing.

Half Life Decay differs from the original and the other expansion packs by having a more puzzle-heavy gameplay. Some puzzles take some time to work through and may cause some frustrations. There are also some who mention the constant “switching” between the two main characters is bothersome and takes away the fun in the single player mode. While the PlayStation 2 version offers splitscreen for cooperative play, the PC version does not.

Half-Life Decay is free for upgrade as downloadable content on their official site, as long as players already own Half-Life. Other expansions include Half-Life Opposing Force and Half-Life Blue Shift.

You begin in a transport vessel. Upon arrival, a security guard will greet Dr. Gina Cross and Dr. Colette Green (the two female scientists who you will be controlling). Proceed forward with one character to the white door; repeat with the other character (default key is “G” for character switching). The security guard will then head to the control panel on the right of the door and allow access.

Proceed forward to another locked door. After both characters arrive the guard will open the door for you once again. Keep going and enter the sliding white doors with a guard inside. The door of the chamber to the right should now be open. Proceed into the chamber with the other character. Once the scan is finished, exit and regroup at the locked sliding white doors. Switch and have the other character be scanned in the chamber. Now the sliding white doors are unlocked.

Head into the elevator. Proceed through more sliding doors to meet a scientist in a wheelchair. Have Dr. Cross go into the scanning chamber. Go through the open door and find the anomalous material cart by pushing a button on the yellow gate. Push the cart into the small opening. The cart lift will malfunction. Use the other character and enter the other scanning chamber. Proceed through the opened door into the “lab” with red stripes on the wall. Find the crowbar stuck in some gears.

Switch to the other player and enter the other scanning chamber. Pick up the crowbar on the ground. Proceed into a room with a large yellow pillar in the middle. Activate the power on the pillar and turn the dial. The experiment goes wrong and a couple of explosion occurs.

Regroup the two characters again by having the upper level character walk down the collapsed silver “tube” onto the lower level. Exit the room through the half open door (duck under) and headcrabs will begin to spawn. Continue on and go down the halls and defeat the zombies and headcrabs you encounter. Keep going until you end up back in the room with the two scientists. After some dialogue, head into the elevator with one of the scientists and the mission will be finished.

After the scene, jump through the opened window into the “old hazard course” and head through the door on the right with the numbers on top. Kill the monsters and continue to proceed forward and avoid falling into the green liquid. Proceed through the vertical door at the end. Complete the second hazard course by jumping and ducking appropriately. Continue through the security door to find a couple more zombies to kill. Proceed until you encounter a ladder leading downwards. Climb down and you will find some ammo and equipment, as well as a health recharger.

There is a small room with a security override button next to the room you are in. Press it and be prepared to have one person dash out of the security door and down the hall. You will see two doors at the end of the hall. Take the RIGHT door with the security sign on it. Switch to the other character and repeat. However, there will be turrets deployed by now, but if you run through it fast enough you will not take too much damage. Now continue on and defeat some monsters. Open the door with the panel on the right. Work through the firing range and kill off any incoming monsters. Once finished, go up the ramp and advance to the next area.

Continue on and drop down the opening into the water. Have one person climb up one ladder and turn the valve. While the fans are stopped have the other person climb the other ladder and into the door. Defeat the monsters in this room and take the shotgun. Activate the lever and the other player will be able to go into the room and have them also take the shotgun.

Head into the next room and kill all the monsters that appear. Continue on into the next room. Have one person hit one lever; the other lever will pop up and the other person will hit it. Repeat until the elevator is at the top. Climb the ladder next to the box and hop into the elevator once a hole appears at the top. In the next room, destroy the turrets with the shotgun and your pistol. Go through the security doors after and kill the zombie. The mission ends here.

First, you will have to find a way to open the locked door to the Control Room. Go in through the other door and unlock the Control Room door from the inside. To do this, you will have to activate the large machine in the small empty room by turning on some computers first. There will be some enemies that teleport in. When you get to a button that you cannot press, smash the wooden planks and you will be able to press the button afterwards. Keep proceeding until you get to another “puzzle”. This puzzle involves hopping on and off several carts at appropriate times. Once the puzzle is completed, move into the hallway where some enemies will be waiting.

Continue until you get to two separating paths that actually end in the same spot. Choose one path (or both) and you will end up at some steps that lead to the surface. Proceed and enter the next building through the back entrance. Open the door with the “Exit” sign above it and the doctor will come in.

The scientist will then tell you to align the satellite dish. Kill all the enemies that teleport in while proceeding to the Satellite Dish room with a blue sign. There will be a wheel in the room. Hold down the use key until the light turns green and return to the doctor. This ends mission #3.

Go through the opened door and smash the glass door. Go through the red doors and continue on into a two-story room where you’ll see helicopters coming in. Continue past the half opened door and through the glass hall into the next room with another puzzle. You will see a bunch of electrical currents and liquids on the floor behind a broken vent. What you want to do here is press a button to open another door which will block the electricity. Once one character is through you’ll end up on the second level of a room. Jump down and clear the enemies that teleport in. Find the security override button to open up the door for the other player to rejoin your group.

Make your way up the stairs into the destroyed lab, kill the enemies and open the double doors with the button. Continue through the hall, ignore the door with the red text on top, and at the end of the all will be several flights of stairs. Proceed onto the walkway that goes over a large pipe and kill the enemy on the other side. Go up the ladder furthest away from the door you came into the room. After climbing the ladder you will see a wheel. Have one player turn the wheel while the other kills the enemies that spawn in. Go down the ladder and up the other ladder. Turn the wheel again like in the other room. Once both wheels are turned, go back to the door with the red text above it from before and it will now be open.

Keep moving down the hall and get up onto the roof of the two-story room you were in earlier. Go into the vent with a red light and get ready for a tough fight. Once it is clear, go through the vent and go through the bottom into a room with some panels. Press the button on the panel next to the health recharger to find some well needed ammo and equipment. Now, have one player go through the door with “DISP. FIELD…” while the other one presses another button in the room to open up a door. Head to the wheel and be prepared to have another tough fight once the wheel is turned. Once the fight is done and some dialogue passes, the mission is over.

After the scene, break open the window to access the control panel which will open double doors. After the scan, continue on and follow the path with the red stripes. In the next hallway there will be a button for you to press to access the dormitories. There will be a fork here with two paths leading to the same place, so pick either one and work through them until the two paths meet again in a small “rest area”. Keep moving and go up the staircase and turn right to encounter a sentry and a trap (line rigged with explosives). Enter the pool area and proceed to break open the vent in the corner. One person will have to stay behind here.

Have the person in the vent proceed and then drop down into a locker room. Regroup the two players and go up the ladder in the locker room. Here, you can either kill the enemies below yourself or blow open the vent wall ahead of you and drop the enemies that show up below to clear it out for you. Drop down to where the enemies were and proceed through another road fork to get to the doctor.

To find the security guard with the access code, retrace your steps and find the library (room with books). Inside you will find the security guard. Once everything is cleared out the guard will begin to move. Just follow him and the mission will be over.

Move through the opened door after the doctor’s speech. You will see a gunfight happening but the path is blocked by a desk which you can destroy by shooting or with your crowbar. Go back to the beginning next to the first “opened door” and go into the vent. Go up the ladder. Find some crates blocking a door and destroy them. Now you can access the blocked door in the room where the “gunfight” happened. Activate the lever and the double doors will be unlocked. Continue on and kill the enemy in the elevator and activate the button to regroup the two characters.

Proceed out the door of the room with the soda machine and drop down on some crates under a ledge. Kill the enemies and continue onwards to destroy the helicopter waiting outside. Make sure to avoid the helicopter when it drops down because it causes an explosion that will damage/kill you.

Alternatively, you can ignore the helicopter and keep moving on. Venture on and there will be a few more enemies and a door that will require a button to open. Have one person stay at the “security” button and the other one press the button further on after the “security” button is pressed. This ends mission #6.

This mission is pretty straightforward, and basically involves lots of enemies to fight. First, go through several doors by opening corresponding valves. From here, just keep killing enemies and proceeding forward. Remember you can always allow the human and alien enemies kill each other first to save some ammo/life. Eventually you will get to a cracked wall with a barrel. Break the crate next to the barrel and move the barrel in front of the crack before you shoot it to reveal the next room. This room simply gives you ammo and health. Head up the ramp to continue on with the mission.

Proceed forward to find lots of enemies for you to annihilate. Soon you will come to another crack but in a fence this time. Again, move the barrel in front and blow open the fence to find more goodies. Be careful when you move on, because you will encounter some exploding enemies which will destroy the floor beneath them. Also, make sure you avoid the steam from the pipes. To turn off the steam, turn the wheel with one player and have the other player turn the OTHER wheel. Go through the door that was blocked by the steam and continue on.

Continue until you find a beacon and some controls. Have one person stand on the beacon. The other player should access the controls and the beacon will move. To clear the obstruction, have the person on the beacon jump on the walkway. Proceed until you find a wheel. Have the other player move away from the controls before the wheel is turned. Go down and press the lever with one player and have the other press the other lever when it rises. After some repetition the mission ends.

Start off by working through a bunch of rooms filled with enemies. Once you reach the beam matrix controls, break the window and hit the button behind the scientist. Now go into the room with the coloured beams and the goal here is to access the “Security Override” button. Have one person (P1) go through the security door, and have the other person (P2) stay with the controls for the coloured beams. P1 will have climb down a ladder and match all large crystals with particular coloured beams to trigger it. Use the reflective “mirrors” to reflect and combine different beams to trigger the crystal. The crystal will glow when they are ready. Once all the crystals are trigged, lift up all the reflective “mirrors” and activate all three coloured beams (with P2). This will start the beam matrix.

The mission will be filled with intensity here, as enemies begin to teleport in from everywhere. Your P1 will have to continue “forward” while your P2 will have to backtrack to the beginning of the level. Your P1 will have to activate the “eye scanning” room to let your P2 back through. Once regrouped, go up the elevator the mission is finished.

Get the ammo and weapons in the two opened doors. In the next part, you will have to protect the beacon (the yellow structure) while the beam capacitor charges. Listen to the scientist’s instructions and turn on the panels when he tells you to (they are located on the walls in several areas and have a red lever). The waves start easy and gets tougher as it progresses, so be careful. Soon an alien ship shaped like a sting ray will appear. Shoot it down with the rocket launcher (of course you can also use your other weapons) and continue to kill the enemy waves that spawn.

Once the alien ship crashes, the ending plays and the credits roll!

Note: A bonus mission is unlocked if you achieve an A rating for both players on every mission.