NEW YORK — Amy Herzog’s beautiful play “Mary Jane” is, at its core, a study of the extraordinary lengths to which a mother will go to fight and care for her child. But the takeaway from time spent at the Samuel J. Friedman Theatre goes beyond even that realization. You leave after 90 minutes with a near-crushing awareness of the unfairness of life, how some moms with a well of parental love inside get to stand in playgrounds on gorgeous fall afternoons while their kids run around and laugh. Meanwhile, others are forced to deal with hospital beds and code blues, the ups and downs of shifting diagnoses, live-in nurses, drug regimens and hospital music therapists who cannot be tracked down in time.

“Mary Jane,” which stars the excellent Rachel McAdams in the title role and was first seen at the New York Theatre Workshop in 2017, is a closely observed play, reflective of the same attention to detail that its title character showers on her child, who has cerebral palsy. The play, to be clear, is not about the kid (who we never fully see), but about his mom and her experience caring first for her child at home and then in the hospital. The writer draws from her own experience with a sick child and as you watch this journey you constantly think to yourself that Herzog must have had that very conversation. They’re all too visceral to have been made from whole cloth.

The most powerful of these scenes involves a conversation with a doctor, played with clinical acuity by April Matthis. The subtext of the scene is that the doctor, leaving room for hope, is gently trying to get the mother to realize the bleakness of her child’s prospects and thus better weigh the risks and benefits. Herzog’s skill as a writer means that you feel the conversation alongside McAdams’ character, you are alongside her on a journey no one wants to take.

Medical dramas are very difficult to stage in a live theater for all kinds of reasons. But director Anne Kauffman (with the help of a simple but emotionally rich design from Lael Jellinek) calibrates this one with an authenticity utterly unfamiliar to hospital procedurals. There is an exquisite grasp here of the feeling that Karen Carpenter was expressing in the song where she sang “Don’t they know it’s the end of the world,” the bizarre disconnect those under familial stress feel from a world that refuses to stop or even slow down. “Why do the birds go on singing,” indeed. But they always do.

Mary Jane is a personal assistant who is struggling to care for her child and keep her job and McAdams has a cheery, round face and infectious smile. The power of the performance lies in McAdams’ ability to deglamorize herself without letting that undermine the formidable, everyday optimism of this character. How could all of this happen to her, you keep thinking, letting the play send your mind spinning as to what life for Mary Jane would have been like without this challenge.

Herzog is writing about a mom who talks to relieve stress and prevent her own breakdown and she shares some of that compassion with a couple of other characters, including another hospital mom, Chaya, played by Susan Pourfar. This carefully calibrated and furiously unsentimental scene, intimate but uneasy between two women of utterly different experiences, will be familiar to anyone who has built an unlikely relationship in a hospital or care home.

Only the last scene, between Mary Jane and a chaplain (Brenda Wehle) does not ring entirely true. It’s not clear that the writer knows what she wants to say about non-earthly matters and thus the scene gets stuck in a rabbit hole.

But life as a parent in this world, loving a struggling kid? Hard to think of another play that understands so well.

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At the Samuel J. Friedman Theatre, 261 W. 47th St., New York; www.manhattantheatreclub.com.

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