Q offers Picard a chance to relive his youth.
After Picard is fatally wounded, he finds himself in a mysterious afterlife with Q, who offers him a chance to go back and change a youthful mistake that led to his artificial heart. Picard accepts, and he finds that changing the past has unexpected consequences. The young man who made the reckless choice turns out to have been an important part of what made him who he is.
The story is a thoughtful examination of regret and identity, and it is one of the most beloved Q stories. It shows Picard as a young man, and as an older man who has to accept the choices he has made. The conclusion is quietly powerful. It is regularly named as one of the best of the entire series.