Alien spacecraft crashes in the Carribbean. Pilot ejects. Alien virus in the pilot (which caused the crash) leaches into the environment – slowly. Over the years the local witch doctors refine zombie powder from the virus and do the whole zombie thing, as memorialized in popular literature. One bloodline of witchdoctors protects and passes down the pilot's gear (alien artifacts) which have some power over the zombies.
Offshore drilling rig hits the main ship which is at the bottom of the ocean. Buckets of the alien zombie virus are released from the ship's medical supply stores, which have degraded and mutated in the hundreds of years of its watery isolation. Oil and virus leaks into the ocean, and make their way to the packed beaches of the not-Virgin-for-long Islands. Teenage kids have sex and are then infected, tearing each other to pieces. A giant zombie octopus comes up from the depths. All hell breaks loose. The oil companies chopper out of Dodge. Local police are overwhelmed. Cruise ships collide and the hapless occupants are thrown into waters infested with sharks, zombies, and zombie-sharks. A beachfront wedding goes waaay wrong.
A small group of people run to the hills, pursued by zombies, zombie-monkeys, zombie-snakes, zombie-crabs, and the like. The group consists of an environmental watchdog group geologist ("The thing is, CaribOil was actually doing everything right.") that happens to be a smoking hot chick, a newlywed couple, the couple's best man who secretly desires the bride, a local cop who is a reasonable match for the geologist, and a couple local kids who know where all the weed grows. They enter into forbidden witchdoctor territory and are nearly killed, but then it turns out that the attacking witchdoctor family is actually protecting them from the giant zombie octopus that has crawled up the mountain.
Along the way these things happen, not necessarily in this order:
The best man's treachery endangers them!
The best man gets his comeuppance!
The lovable stoner locals are killed, sad.
The honeymooners have sex and are killed!
The geologist and the cop have sex and live!
The witchdoctor gives them all some crazy weed laced with alien blood.
A zombie monkey talks! It says, "fuuuuck youuuuu."
The alien spacecraft explodes, taking the oil rig with it!
An alien medic-robot is torn apart by a zombie squid.
The heroes are caught on a rope bridge with zombies coming from both sides!
The cop runs out of ammunition.
The geologist explains that she knows her way around dynamite from doing seismic testing.
The witchdoctor sacrifices himself to lure all of the zombies into an unfinished soccer stadium that can be collapsed onto the zombies, trapping them long enough for them to be burned.
The keys to the oil rig are in the building with the zombies!
Nobody knows how to fly the old chopper!
"Now all a' sudden you wan' know how to make Voodoo. It's typical, isn't it, the white man all a' sudden need ever' t'ing right quick…." This while the zombies are approaching!
A zombie-bird looks down on a fresh new uninfected island from up on high.