I'm guessing...
1. Alyx has gone emo (kinda)
2. Near the beggining, the helicopter is on the way to the borealis. You crash, and are attacked by Combine assassins.
3. You are incapacitated.
4. Alyx kills them all.
5. She takes armor from one of the bodies and dons it, representing the mental separation between her and everyone else.
6. You free the rebels on the borealis.
7. Judith is discovered near-death.
8. A large battle ensues, and Alyx nearly kills you at the end.
9. She realizes how she is just making everything worse as a cold-hearted killer, and throws the helmet away, but keeps the rest of the armor.
10. Mossman dies.
11. CHELL IS ON BOARD OMG
12. She demonstrates the portal gun.
13. Another Epic battle
14. ...with CRAB SYNTHS!!!
15. You discover whatever the cargo of the borealis is.
16. You destroy it.
17. Whatever destroying it did, it kills all combine on Earth.
18. Kleiner and Magnusson create a teleporter (again) that can take you to Combine worlds to free them from combine rule.
19. THE END of the episodes
20. Development of HL3 begins
