Will Reeve describes moment he felt “alone” after deaths of Christopher and Dana Reeve

While most of the world knew Christopher Reeve as Superman, to his three children — Matthew, Alexandra and Will Reeve — he was simply their beloved dad.

The three siblings watched firsthand as their father went from movie star to pioneering activist for spinal cord injury research after a near-fatal horse-riding accident in 1995 left him paralyzed from the neck down at the age of 42.

Then, in 2004, Christopher Reeve died unexpectedly due to heart failure.

In addition to his children, by the actor’s side from his accident to his death was his beloved wife Dana Reeve, mom to Will Reeve and stepmom to Matthew Reeve and Alexandra Reeve.

Less than one year after delivering a eulogy at her husband’s funeral, Dana Reeve, a non-smoker her entire life, was diagnosed with Stage 4 lung cancer.

She died seven months later on March 6, 2006, at the age of 44.

“Despite the love and security that my siblings provided me, and my family provided me, and my adoptive family provides me, that was the moment, March 6, 2006 … I’ve been alone since then,” Will Reeve, who was 13 when he lost his mother, said in a new documentary, Super/Man: The Christopher Reeve Story, that explores the actor’s life.

Prior to her death, Dana Reeve made sure Will was taken care of, arranging for him to live with the family of his best friend.

His older siblings also dropped everything to help him. Alexandra was a law student at the time and Matthew a producer.

In their conversation with ABC News’ Diane Sawyer, Will Reeve, now an ABC News correspondent, posed a question to his siblings that he had never before asked them — did people worry enough about them after their father and Dana Reeve died.

“I don’t think I’ve ever thought about that,” Alexandra replied. “The job at hand was keeping things going, keeping us OK, keeping everyone OK, honoring them in the right way, setting you up for success.”