Writing Prompt: The Countdown

In the movies, televison shows, and many action-adventure works of fiction, characters race against a clock to ensure that action is taken before it is too late.  Something will happen at a certain point in time and characters either have to prevent it.  A countdown builds tension as the action progresses.  A great example of a countdown story is the television show 24. 

For today's writing prompt, create a short story that involves a -

countdown.