Passengers in Time and Space

It was just an ordinary day when 136 alien spacecraft suddenly descended on the cities of the world making no attempt to communicate with mankind. Scientists were sent in to investigate and the ships let them inside before taking off with them still inside. Now 50 years later the ships have returned to a world that is dying from environmental catastrophes, and solving the mystery of where the spacecraft have been, and where their passengers have been unaged all that time, could be the key to solving the mystery of whether we are alone in the universe, and possibly also the key to saving our own species and planet.