Half-Life: Decay is set at the same length of time as the original Half-Life. Dr. Gina Cross and Dr. Colette Green are members of a different team in Black Mesa than Half-Life's protagonist, Gordon Freeman. They are responsible for monitoring the equipment which Gordon uses during the experiment, and are in turn monitored by Dr. Rosenberg (who also appears in Blue Shift) and Dr. Richard Keller.
Cross and Green deliver the test sample to Freeman just before the "incident" occurs; like Freeman, they are forced to fight for survival in the aftermath of the resonance cascade. Cross and Green help Rosenberg reach the location he is later found in by Barney Calhoun in Half-Life: Blue Shift. They then proceed to prepare a satellite for launch (an event seen in Half-Life) and subsequently use the satellite in tandem with ground-based equipment to significantly weaken the effects of the resonance cascade. During the conclusion of the game, both characters are caught in what is explained as a "harmonic reflux" by Dr. Keller. It is during this sequence that the voice of Dr. Rosenberg can be heard warning that he "can't keep the portal open much longer," alluding to Calhoun's coinciding trip in Xen during Blue Shift. Dr. Cross and Dr. Green then return safely and Dr. Keller congratulates them on their success. It still remains unclear whether the doctors managed to survive the nuclear explosion that occurs at the end of Half-Life: Opposing Force.