The crew of a lone naval ship try to save humanity when they discover a deadly virus has wiped out most of the population.

The crew faces a new foe targeting Rachel and her research, leading Chandler to engage in a series of dangerous strategic maneuvers.

A mission to help Rachel with her vaccine trials leads Chandler and a small team into the jungles of Nicaragua, where they tangle with a former drug kingpin who will stop at nothing to maintain his power over his oppressed society.

Panic spreads when Danny Green catches a mysterious illness that might be the virus. Doubts rise about Rachel and her failing vaccine trials as Chandler deals with a crew whose uneasy state grows.