After a vagabond takes shelter in an abandon building, he discovers an alien invasion and must thwart it before the walls come tumbling down.

Thanks for the comments. Your questions are good, but I don't have it all worked out yet.

Hunger is an internal conflict for them, being that they hadn't eaten in days, now their trapped, so cannibalism becomes a potential issue. Multi pro isn't necessary. The aliens probably were already there, underground.

I think this sounds like the beginning of a great idea.

I would add to Nir's comments: WHY are they trapped in an abandoned building (why can't they just run away)? Why are they vagabonds? Are they mentally ill? Homeless due to financial problems? "Hobos" by choice of lifestyle?

Also, yes, we need ONE main protaganist and the rest are his/her supporting cast. Finally, why in the world are the aliens so focussed on one building? Perhaps, the government has locked the aliens in the building without realizing it was "occupied" and now it's an "Alien" the movie scenario, but on Earth.

Maybe that introduces a ticking bomb: the government is set to blow up the building at some immediate point in the future. So time is also an element?