Two gifted young women whose best work will be in other films are the reason to see this one. Carey Mulligan, so enchanting in last year’s An Education, plays a pregnant teenager in “The Greatest,” written and directed by newcomer Shana Feste. Both show a great deal of promise in this sincere but uneven film.

Mulligan plays Rose, who has one perfect moment with Bennett (Aaron Johnson) before he is killed in a car accident. They were both seniors in high school who had watched each other and waited for glimpses of each other but hardly even spoken until the last day of school when finally they work up the nerve to speak to one another. And then, suddenly he is gone, and she is pregnant.

Bennett’s parents, already dealing with a lot of dysfunction, are devastated by the loss and driven apart by it, too. Allen (PIerce Brosnan), a professor of mathematics, is rational and keeps his feelings inside. Grace (Susan Sarandon), is emotional. In one shattering scene, she is in the bathtub and he hands her a bell to ring when she misses her son. She rings it immediately, insistently, harshly, making it clear that her pain is deep and permanent and cannot be confined. She is obsessed with the 17 minutes between the crash and his death. What was he thinking? What did he say? Did he suffer? But the only one who knows is the man from the other car, who is in a coma. Grace visits him, reading aloud, monitoring his care.

Rose, who has nowhere else to go, moves in with Bennett’s family. But it takes a while for each of them, grieving separately, to find a way to reconnect as a family.

Surprisingly, Feste is best with the older generation in the film. Brosnan and Sarandon are the real center of the story and their characters are the best defined and the most compelling. Johnny Simmons (“Jennifer’s Body”) and Mulligan do their best with roles that are both under- and over-written. Simmons is the younger brother, a recovering drug addict, whose primary job in the movie is to remind his parents every day that they are left with the troubled son instead of the one they were proud of. But he is stuck with a distracting sub-plot about a relationship with a girl (Zoe Kravitz) from his support group. The problem with Mulligan’s character is Feste’s view that in the midst of terrible grieving and dysfunction, the repository of all wisdom and imperishable goodness resides in a pregnant teenager with a disastrous home life but an adorable dimple. This leaves a blank space that unbalances an already-unwieldy story but leave us looking forward to seeing how Feste learns from this film do to better next time.