Five scientists are transported to an alien spacecraft in Earth orbit. There, they are met by a humanoid referring to himself only as "The Alien", who explains that he is the representative of a world orbiting a sun about to go nova. Needing a new world to inhabit within the next 35 days, yet prohibited by their moral code from killing intelligent life, The Alien provides each of the five with sets of three capsules in a clear, round, hand-held case. Each capsule is capable of destroying all human life within a 3,000-mile diameter, with the expectation that humanity will use all the capsules, obliterating itself, leaving the Earth free for alien colonization. The capsules' clear containers can only be opened by the thought waves of the person to whom they were given. Once out in the open, the capsules inside can then be used by anyone, but only during the next 27 days, after which they become inert.

References:
https://en.wikipedia.org/wiki/The_27th_Day