In the classic star-crossed romance, a crime forces a choice between youthful passion and family loyalty. For Romeo and Juliet, it’s the murder of Mercutio; for Tony and Maria, it’s the knife fight between Sharks and Jets. This is how tentative adolescent love turns into high drama. Parents don’t object to boyfriends or girlfriends for being taciturn or tattooed. They object because of a blood feud.

In “You Against Me,” Jenny Downham renews this formula by making the crime that divides her characters an alleged date rape. The girl who says she’s been assaulted is Mikey McKenzie’s sister, Karyn; the boy she accuses is Ellie Parker’s brother, Tom. As if that’s not enough to doom the smitten Mikey and Ellie, Ellie is her brother’s key witness — she was the person who was there the night he brought Karyn home and got her drunk.

When Mikey and Ellie first meet, her wealthy family is throwing a welcome home bash for Tom, who has just been released from pretrial detention. Crashing the party in pursuit of revenge, Mikey doesn’t let on to Ellie who he is, which allows them to be drawn to each other. If that sounds creaky, ask yourself if Romeo and Juliet had it any better.

Mikey and Ellie aren’t quite as arresting as Tessa, the fierce protagonist in Downham’s heart-­stopping first novel, “Before I Die.” But they’re vivid enough to make their shared dilemma come alive. Mikey, 18, lives in an English housing project with his two sisters and alcoholic, emotionally AWOL mother. When a cop visits, Mikey courts her with a cup of tea, then panics: what if she asks for a biscuit? “Weren’t children supposed to have nice things to eat in their homes?” Meanwhile, Ellie, 16, has the makings for a fancy picnic in her kitchen. But her father has shrunk from her since she reached puberty — “it was a slow retreat,” as Ellie puts it — and her mother won’t accept her memories of the night of the alleged date rape.

Increasingly, what Ellie has to say complicates the family’s effort to defend Tom. Downham’s strength here is in allowing for plenty of moral ambiguity. Mikey’s mother wonders why her daughter Karyn got so drunk at Tom’s house and whether it would have been better if she’d never encouraged her daughter to go to the police. Ellie wonders how Tom could have known Karyn wanted to have sex with him, as he claims, given the drunken state she was in.

Most daringly, especially in contrast to the often simplistic portrayal of situations like these, Downham also shows Mikey and Ellie acting on their own dark impulses. Mikey wants to protect Karyn, but in his own romantic escapades, he can’t even be bothered to undress a girl he sleeps with. Ellie wants Mikey to play it straight with her, but then she lures him into her bedroom to set him up. Their imperfections heighten the ambiguity. It helps, too, that Mikey and Ellie use the same awesome slang that the teenagers use in the British version of the comedy-­drama “Skins.” “Do you fancy her?” is so much better than “Are you guys going to hook up or what?”

As the story builds to its tumultuous conclusion, Mikey and Ellie have to make the inevitable choice between new love and old allegiances. But because of Ellie’s role as witness, her decision is the more fraught. She’s in a position no one would wish for. The ending falters a bit, but that too may be inevitable. The hard question at the heart of this book — what would you do? — doesn’t have a single answer. For her young readers, Downham frames it remarkably well.

Originally posted on the NY Times by Emily Bazelon.