Aftereffect by T. John

After global nuclear warfare, humans have moved underground to survive-though their safety comes with a time limit. Radiation, nuclear winter, and even extraterrestrials all push the countdown closer to its end. Meanwhile, a small group from another planet work to complete their collection of Earthly artefacts and get back home before being discovered. Over time both stories will crossover as each group looks out for their own best interests to not only survive the present, but also make way for the future.