Gina and Colette are forced to defend the displacement beacon against a variety of attacking Xen creatures, including an Alien Craft. In the midst of the battle, 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. Gina and Colette then return safely and Dr. Keller congratulates them on their success.
The resonance reversal presumably had the effect of severely weakening the resonance cascade. It still remains unclear whether the doctors managed to survive the nuclear explosion that occurs at the end of Opposing Force.
This Chapter contains examples of:
- Final Battle: Gina and Colette's stand against the Xenian forces while they defent the displacement beacon.
- Level in Reverse: The final mission is an inverse of the last level of Half-Life, where Gina and Collette have to protect the Resonance Beacon from waves of Xen aliens, while charging three huge Xen crystals to finally close the dimensional rift between Earth and Xen.
- Unique Enemy: This level is the only time in the whole series where you actually fight one of the Alien Aircraft (nicknamed "Manta Rays" by fans). They do appear frequently in the other three Black Mesa era games, but only in the background, in scripted sequences, or as props, not as enemies.
