Dr. Gina Cross and Dr. Colette Green are members of a different research team than Gordon Freeman at Black Mesa. They monitor the equipment that Freeman uses during his fateful experiment which opened up the portal to Xen. They deliver the test sample for Freeman's experiment to him, unknowingly putting events into motion, with them having to fight for their survival, just like him. They are subordinates of Dr. Rosenberg (seen in Half-Life: Blue Shift) and launch a satellite into orbit (seen in Half-Life) to weaken the resonance cascade. In the conclusion, the player hears Rosenberg say that he "can't keep the portal open much longer", referring to Barney Calhoun's trip to Xen seen in Half-Life Blue Shift. It is unknown if either of the the protagonists survive the nuclear explosion seen in Half-Life: Opposing Force.
Unlike the rest of the games in the Half-Life franchise, Decay is meant to be played co-operatively in split-screen multiplayer, although it can be played by one player swapping between the roles of Dr. Green and Dr. Cross.
Many reviews for Half-Life: Decay were in favor of it, praising the design of the puzzles requiring players to work together, as well as the way in which the ranking system discourages players from competing against each other. While critics like this new addition to Hal-Life, it was also criticized for being tedious, with one member having to wait for minutes at a time, while the other can lag behind, not understanding what is going on.
While the original version of Decay is on the PS2 version of Half-Life, there is an unofficial port of it to the PC as a mod for the original Half-Life that takes advantage of being on the PC, with updates graphics and internet play.