Now that Michelle fears the worst, after seeing the crazy infected lady in the car outside bash her head against the bunker door, the three survivors get on better and make happy families.
The air filtration system needs to be rebooted - the only way to do this is to climb through the vents to another part of the bunker. Michelle is the only one that will fit, also giving a clue as to why she is there. She climbs through the pipes and resets the engine, but notices on a small window at the top of the smaller shaft the word 'HELP', and a bloodied earring used to scratch it. Later she shows the picture of Howard's daughter (who was wearing the same earring) to Emmett, but Emmett says it's not his daughter - it was a girl from his school who went missing a few yeas ago.
Michelle and Emmett scheme to make a suit and gas mask so they can escape outside. Howard is soon onto them though and after confronting the pair Emmett takes the blame. Howard shoots Emmett in the head, then dumps his body into a vat of acid.
A traumatised Michelle then finishes the suit, but when Howard finds it she fights back, kicking over the acid where Howard slips and falls head first. It reaches the cables and starts a fire. She uses the ventilation shaft to get out, but Howard is still around trying to stop her, until she manages to kick him free.
She dons the suit and mask, and uses the nitrogen canister to open the smaller shaft window where she gets out just in time. Outside everything looks calm, and she checks that the air is clean ... until the bunker explodes - and just then an alien spaceship in the distance approaches and drops off some alien monsters to finish her off, and a cloud of green toxic gas!
She puts back on the mask and hides outside the bunker, and takes the keys from the old lady who killed herself, but to get away from the monsters she picks Howard's truck instead. The spaceship pulls near, and tentacles reach down to pick up the truck. Using a handy bottle of whisky, she makes a Molotov cocktail, and with a good arm throws it towards an organic 'mouth' or heart part of the spaceship. Once inside it explodes, and the truck is dropped. She makes a getaway.
She turns on the radio to hear about survivors in Baton Rouge, or combat vets needed in Houston, as she approaches a signpost. She decides to fight the good fight but as she travels the road to Houston lightning illuminates an alien spaceship waiting in the clouds.