This should be something that you have reason to believe the company will value. If there's a story involved, tell a very abbreviated version, leaving out peoples' names and other irrelevant details. The interviewer is only interested in what obstacles you overcame or how you served your employer at the time. Don't go overboard with this explanation. Stop just short of telling everything and leave the interviewer room to ask some follow-up questions.
People are naturally curious and interviewers are likely to take the bait.
If you can lead the interviewer down your path, you've gained a measure of control over the situation. This ball is in your court. Do practice giving your answer conversationally. This is a critically important question - have a specific answer. It's the place where you put your research about the company into the conversation and explain why this is where you'd like to be.