He is an alpha predator, feared by all life on the planet Challenger. His pincers can crush steel, his razor-sharp mandibles can shred a hundred pounds of flesh in seconds, and his armor carapace is capable of deflecting anti-tank projectiles. His place in the ecosystem is reflected in his name: Caesar. A beast of incredible size, he has a major appetite to satisfy. And now, he has a new item on the menu: humans.

When Corporal Richard Carson and his team entered cryosleep, they did not expect to awaken in two days for a whole new mission. An island research facility on the ocean planet Challenger has gone dark. Efforts to make radio contact have been unsuccessful.

When Carson and his team set down on the planet, they find the place in ruins. The radio tower has collapsed, the research station ripped open, the surviving staff in hiding.

As seismic activity shakes the side of the planet, the truth reveals itself. A pack of giant alien crustaceans, led by the gargantuan Caesar, storm the island. Following a devastating attack, the marines take defensive position in the remains of the station.

With their numbers depleted, their dropship destroyed, and no means of radioing for help, they are left with no choice but to fight Caesar and his pack of killer crabs.