Honestly, the game's story is very average for its genre. Maybe 3 or 3.5 stars. I am giving it 4 though for two reasons: for starters, it is probably the most stable, bug-free gaming experience at launch I've had in recent memory; secondly, I actually remember Dead Island 1 and this game's story, gameplay, and overall cohesiveness are wayyyyy more fleshed out than its predecessor's (no pun intended).
Dead Island 1's story was very lame, very stretched out, and akin to the whole "ancient burial grounds" trope every show seems to have an episode referencing. I enjoy Dead Island 1, it is one of my favorite "cozy games". You can turn your mind off and just start hackin' away. But Dead Island 2 dares to go a step further. Other than not having wacky weapons like The Ripper or drivable cars, Dead Island 2 is a massive step up from the original game in terms of gameplay and story. Although it aims to satirically paint LA as a city overrun by brain-dead zombies metaphorically in a literal display, it does fall short in its execution, though I can see that it tried.
It doesn't do anything new and to any average gamer Dead Island 2 falls short as a game on its own, to a Dead Island fan this is expectations exceeded.
I would genuinely implore anyone who has played the first Dead Island back in its hay-day and is disappointed in Dead Island 2 to revisit the first game and then compare them.
Dead Island 2 is a fantastic game for its time, and unfortunately for many, that time is 10 years ago.