A year has passed for the community of survivors. Ian and his team are still making dangerous forays to the ravaged surface in search of food and medicine, forced to go ever further each time. But one day, just as they're about to be killed by one of the exterminator robots that constantly patrol the city, they're saved by an old acquaintance not even Ian still hoped to see again: SAM, the strange robot that seems to want to protect the boy...