How mad are you allowed to get at a schizophrenic?

That’s the premise driving “Broken Diamonds,” a dramedy that may start “crazy cute,” but which doesn’t back away from the ugly truths about mental illness.

“Your sister didn’t ruin your life, her illness did,” a therapist lectures Scott Weaver (Ben Platt of the “Pitch Perfect” movies).

“Same thing.”

We meet Scott at his going away party at the restaurant where he’s quit his job. He’s packed his place, has a “For Sale” sign in his Mazda and makes daily calls trying to locate where his long-ago-ordered passport is.

He’s moving to Paris, he tells everyone. He’s going to be a writer.

That’s when he gets a call. His father’s died, Dad’s second wife tells him.

“Pick up your sister on the way.”

He does. But Cindy (Lola Kirke of “American Woman,” “Mozart in the Jungle” and “American Made”) won’t get out of the car, won’t look upon the corpse or talk to Dad’s widow (Yvette Nicole Brown).

Scott collects a few effects, takes the deed to the house he and his sister inherited and drops his older sister off at the mental hospital that’s been her home for a pretty long time.

Cindy’s bubbly, a tad manic and full of ideas about having Dad’s ashes “turned into a diamond,” totally a thing. “If I gave it to you, would you pawn me?”

Scott? He’s focused on getting out the door and out of the country. Naturally, that’s when he gets the second call. His sister’s “issues” at her hospital have gotten out of hand. She’s evicted. There’s another place that will have room for her…in two weeks.

Before he’s gotten her home she’s thoughtlessly damaged the car he’s desperate to sell. Before he can get a bead on getting a quick sale of their father’s house, she’s set his apartment on fire, torching the passport she just signed for on delivery. He finds her staring at the lamp she’s ignited.

“Did you not know if this was real?”

Counting the pills in her various prescriptions, he may get the answer he doesn’t want.

Kirke plays Cindy as mercurial and cute — but a walking collection of triggers whose delusions include going to the party of a high school friend (Debs Howard) who became an actress, to “finding a job” to live independently — to living independently.

Scott, “guilt tripped” by her therapist (Catherine Lough Haggquist) indulges her, but Cindy can’t fake sanity long enough to get through a job interview.

“I can’t stay here, no no no no no no…”

Irrationality is the rule.

Most of our sympathies are directed at Scott, with flashbacks showing just a hint of the horror Cindy experienced, and of growing up in a house where one child has taken ill and sucked up all the stability and happiness as she did. But Scott’s either developed a martyr complex or simply decided he deserves to run away, and Platt makes these diagnoses understandable, defensible and still not very attractive.

There’s an honesty to Steve Waverly’s script that makes its light moments irritating, its darker ones heartbreaking. A soulful, broken-hearted soundtrack (songs by Julia-Jean Baptiste, Strong Asian Mothers, Nathan Reich and Mia Dyson) underscores the sadness of it all, a life lost, lives around it shattered.

We immerse ourselves in the story and hope for the best even as we expect the worst. All we really know by the end is what Scott’s journey is designed to teach him. He’s allowed to get mad at Cindy. He just has to get over such furies quickly and realize it’s the illness he hates.

MPA Rating: PG-13 for thematic elements and a crude gesture

Cast: Ben Platt, Lola Kirke, Debs Howard, Yvette Nicole Brown

Credits: Directed by Peter Sattler, script by Steve Waverly. A FilmRise release.

Running time: 1:30