Mikhail Rasputin caused the death of the Morlocks back in X-Men #293, and in this issue we see that Thornn initially saved herself by dashing into a tunnel before the sewers were flooded, drowning everyone else.
During her escape, she found Marrow had survived as well. But then the waters caught up to them, and Marrow was swept away.
From this flashback, we move to the present, where Cable and Domino are going on a date. Yes, a date. Cable actually wants to do something other than complete mayhem. The date itself offers some interesting character development (Cable likes Frank Sinatra, e.g.), but it goes sour when Cable mixes up facts about Domino with facts about Copycat (who had been impersonating Domino for quite some time).
The date is interrupted when Thornn shows up. The first thing they do is punch her a few times (even though Thornn specifically says she’s not there to fight).
Thornn says the Morlocks are actually still alive. Cable and Domino help her figure out that, no, they are in fact dead.