Derek (Patrick Dempsey) was missing all through-out episode 20 of "Grey's Anatomy" season 11, and fans thought he was dead when his wife Meredith (Ellen Pompeo) caught wind of a plane crash. The episode ended with cops arriving at the Shepherd home but the trailer for episode 21 proves that not only is Derek alive, he is also a true hero. 

Episode 21 titled "How to Save a Life" will reveal exactly what happened to Derek on the way to Washington.  As the promotional trailer shows, he's not able to make it to his meeting because of a violent car accident that unfolds right in front of his eyes. One of the vehicles in front crashes and flips over, involving more vehicles in the wreckage, eventually hurting more people. Derek, being the good Samaritan doctor that he is, leaps into action to save the lives of those hurt in the wreckage. 

From what the trailer reveals, Derek treats several teenagers seriously hurt from the crash. It also appears that actress Savannah Paige Rae's character may be one of them. Rae is most notably known for portraying Sydney Graham in the NBC series "Parenthood." 

"I don't let people die," Dr. Shepherd promises to one of the victims lying on the ground, covered in blood.

The trailer then ends with one of the crashed vehicles suddenly catching fire. 

Most of season 11 was about addressing family issues, with flashbacks of a young Dr. Ellis Grey and Dr. Webber, Meredith dealing with her half-sister Maggie Pierce, and Meredith struggling with Derek's need for a change in pace and environment. Perhaps this is the episode that will remind Derek where his home and family are. 

The episode is written by series creator Shonda Rhimes and is directed by Rob Hardy. The title "How to Save a Life" will also tug at long-time fans of the series since it is also the title of what many perceive as the unofficial "theme song" of the series since season 1.