Set during the events of the first game, the players take control of Colette Green and Gina Cross, two doctors working at the Black Mesa Research Facility along with Gordon Freeman. After the two help set up the experiment with the Anti-Mass Spectrometer, the experiment ends up triggering the Resonance Cascade and begins teleporting alien creatures into the facility. With the help of Dr. Keller, Cross and Green go to work to find a way to shut down the dimensional rifts and stop the horde of invading aliens.
The game requires the players to cooperate in order to solve puzzles and complete tasks, with each player being ranked at the end of each stage depending on weapon accuracy, enemies killed, and damage taken. Decay was released exclusively for the PlayStation 2 and came bundled as part of the port of first Half-Life. As such, it's the only game in the franchise to have not seen an official release on the PC. However, dedicated fans were able to port the game to Windows as a Game Mod and added the ability to support online multiplayer.
Half-Life: Decay contains examples of:
- 100% Completion: Completing every mission with "A" grades rewards the player with a bonus "Aliens" mission, where you play as a pair of Vortigaunts sent to retrieve Xen crystals taken by Black Mesa.
- Action Girl: Drs. Gina Cross and Colette Green.
- Ascended Extra:
- Dr. Rosenberg from Half-Life: Blue Shift appears at the beginning, and he accompanies you in two of the missions.
- Gina Cross was the holographic trainer from the Hazard Course.
- Continuity Nod:
- The mission "Domestic Violence" takes place in Level 3 Dormitories, and you can find Gordon Freeman's room in one of its wings.
- At the endgame, Gina and Colette get caught in a Harmonic Reflux, and the player can hear Dr. Rosenberg calling to Barney Calhoun through a Xen portal.
- The bonus mission takes place in the same parking garage from the end of Half-Life: Opposing Force, including the truck where the Black Ops' nuke would be set off.
- Doctor, Doctor, Doctor: One of the first security guards Collete Green and Gina Cross meet on their way to work greets them with "Good morning doctor, [beat] doctor."
- Foregone Conclusion: At the end of the uplink mission, Dr. Rosenberg opts to stay and wait for the military to arrive, while Dr. Keller takes charge for the rest of the game. This primarily explains how Rosenberg ended up where he did in Blue Shift.
- Game Mod: The game was ported to PC by dedicated fans.
- Genius Cripple: Dr. Keller is bound to a powered wheelchair.
- Mission Control: Dr. Keller acts as this from the chapter "Resonance" onward, using the radio link in your suits to guide you.
- Noodle Incident: When you first meet Dr. Keller, he complains that Gordon Freeman is running late...again."Honestly, I do not understand what Kleiner sees in that boy."
- Shoot the Fuel Tank: Unlike Half-Life, you don't get the rocket launcher until the final level, and the only way to down an HECU Osprey is to shoot out its two propellers with the magnum.
- Split Screen: Decay's method of display.
- Stop Poking Me!: Just like in the first game, VOX can be poked fun at in the PC Mod's Hazardous Course chapter.
- Teamwork Puzzle Game: Several missions require one player to hold or move something while the other player passes by.
- Two-Keyed Lock: The designers' idea of co-operative gameplay was to include puzzles that require two players to work synchronously to be solved efficiently, such as turning switches on opposing sides of a room simultaneously.
