: - Locke and his team are sent to Sedra to investigate a terrorist attack that
: will be perpetrated by an Elite (and possibly others) (2:07)
: - Locke stops the Elite, but not before he uses an unknown biochemical weapon
: (2:08) and it infects the people of Sedra (1:42)
: - ONI and the Sedrans go to the source of the weapon (the jettisoned piece of
: Installation 04) to find a way to stop it
: - I'm not entirely convinced this biochemical weapon is the Flood. The Flood
: are too big of a force to be harnessed as a weapon and it only infects
: humans. I believe it could be Forerunner engineered; originally used to
: kill the humans during their battles 100,000 years ago. Sealed away on
: Installation 04 for millennia, only to be rediscovered now. The scene at
: 1:17 could be the effects of the biochemical weapon, rather than Flood or
: Lekgolo posing as a human figure.
: - I believe Escalation 11 and 12 tie into Nightfall. The timing of the
: issues is right, the descriptions detail a bio weapon being harnessed by
: the Covenant, and these two issues seem irrelevant to the rest of
: Escalation's storylines. When the descriptions for these two issues were
: first released I was confused, but now the pieces are coming together.

Still think the Flood is the ultimate big bad, but I think your estimation of the first act is spot-on.