“Riding the Bus with My Sister,” starring Rosie O’Donnell and Andie MacDowell with Anjelica Huston directing (Sunday, May 1, at 9 p.m. EDT/PDT on CBS), is inspired by the autobiographical book by Rachel Simon about her sister Beth.

Beth is a vibrant, creative and willful woman who happens to have mental retardation. She lives independently, receiving support from a team of professionals and a network of friends and mentors — the city bus drivers.

In the years before she discovered the buses, none of her attempts at employment worked out. During one of the periods of unemployment, she fell in love with bus riding, and made a decision that this was the calling to which she would devote her life. Indeed, Beth turned her daily schedule into a job of sorts, performing small favors for her favorite drivers.

Partly out of curiosity, partly out of a sense of obligation to her sister, and partly out of the guilt of feeling like a “bad sister,” Rachel accompanies Beth on these rides and watches her interaction with the patchwork group of personalities that make up the city’s public transportation force.

As Rachel accompanies her sister on the buses, she discovers the reason for her sister’s affection and devotion to these drivers — these people Beth deems “cool.” It is apparent in their open demeanors and good humor, and their ability to accept life as it comes, one passenger at a time.

Beth’s determination to live her life in her own way, as well as the optimism and grace of her bus driver friends, encourages Rachel to reexamine her own way of doing things.

“My big journey with Beth,” said Rachel, “has been coming to the realization that I can’t control my sister. To love her does not mean to control her. … People with disabilities have the right to make choices about their own lives.”

Interviewed on the film’s set,

O’Donnell was asked to explain why her character, Beth, rides the buses nine hours a day, six days a week.

“You have to understand,” said

O’Donnell, “that before she started riding the buses, [Beth] lived in a group home. She didn’t see anyone except other mentally challenged people.

“One day she decided just to go out there and get on the bus and see what would happen. And she found a whole new world out there, a whole life, a whole family.”

Director Huston said, “The irony is that Beth — the developmentally challenged character — is light years ahead of the suave Rachel in terms of how she deals with life, her capacity for love and understanding — to say nothing of sense of humor. We can all learn a lesson from Beth!”

Whether this Hallmark Hall of Fame adaptation can live up to Simon’s outstanding book is yet to be seen. As with all such projects, some of the story has been changed or streamlined.

In particular, when the movie Rachel and her brother leave their mother’s house, and Beth stays behind, the flashbacks stop. In fact, some very hard experiences occurred, which were traumatic for Beth and Rachel, and, out of concern for the family, the filmmakers decided to omit that material.

In addition, Rachel is a fashion photographer in the movie, “but [I’m] a writer and professor in real life,” the author said. “Andie MacDowell also looks a lot better than I do in the morning — not to mention throughout the day!”

The film portrays both Beth and the “struggles of the special sibling relationship in a more realistic way than we usually get to see in film, and presents bus drivers as the everyday heroes that I now know them to be,” Rachel said.

“The movie highlights some of the main themes from my book: Beth’s right to live her life by her own choices, the importance of public transportation for a fully independent life, the essential need for friendships in the community, and the challenges and rewards of the sibling bond,” Rachel said. “The fact that the filmmakers kept these themes amazes and thrills me.”

So watch the TV version, then read the extraordinary book. (The paperback edition of “Riding The Bus With My Sister” — now in its ninth printing — published by Plume, is available in bookstores, $14.00.)