Title says it all.
You receive a call to your phone, or this person approaches you when you're about to enter your house. He confesses. He murder someone. You'd ask for details? You think its better not to press for details? Did he do it for the joy of killing? Had him personal motives that forced him to do so?
This buddy is in a bad situation, but someone can save him. You. All he needs is you to fake an alibi. Lie in court.
What would you do? Are you prepared to face the choice?