I'm sorry, but barely anything happens in this movie except for some, to be fair, really well-choreographed fight scenes. But these fight scenes and intriguing plot elements just fall flat because the story is relatively empty and the characters for the most part have no layers, transformation or arc. Ironically the most interesting or layered character is the doctor, and even then it's like they just touch the surface of him. In the end, the main cast quite literally come back to the setting in the beginning with no payoff except for the fact that a family member of theirs died, nothing else has really changed.
I understand that this is supposed to be just a simple and fun apocalypse flick, but your main characters should still have some form of development and layers to them regardless, the characters here are very monotonous and one-dimensional, and there isn't much of a story, just a plot.