Four Minutes Twelve Seconds, Trafalgar Studios

Teenagers lie – that’s nothing new. But are the activities they’re concealing from anxious parents in this oversharing digital age more extreme, more likely to define their lives and those of the people around them? James Fritz’s 90-minute debut, the first of two Hampstead Downstairs transfers to Trafalgar Studios, dives headfirst into that murky paranoia, with dramatically mixed but thought-provoking results.

Di (Kate Maravan, pictured below right) is shocked when 17-year-old son Jack comes home with blood on his shirt. Husband David (Jonathan McGuinness) tries to fob her off with cover stories, but eventually spills the real one: Jack was attacked by his ex-girlfriend’s brother after Cara (Ria Zmitrowicz) claimed he sexually assaulted her and posted an explicit video online – revenge for her dumping him. Both parents automatically defend their boy, but as deceptions tumble like dominoes, their cosy liberal assumptions are obliterated.

Fritz sets out to demonstrate that there’s no such thing as simple, impartial truth, so we never meet Jack. Instead, his identity morphs as we gather information that conflicts with and complicates his version of events. Though thematically effective, this device grows frustrating, forcing everyone to act illogically in order to work around his absence. Contrivances pile up, pulling us away from the grounded, wittily observed domestic portrait as characters are contorted in service of theoreticals. Moving into the abstract lessens the emotional impact.

The debate is gripping, however. Fritz’s deliberate language shows this isn’t an individual problem, but an entire culture we urgently need to address. David dismisses and victim-blames Cara, confirming her fear that because of her gender, class and appearance she won’t be believed over Jack, whose impressive grades offer him a future beyond west Croydon. But is his behaviour so terrible, wonders David, when everyone is casually exchanging explicit material? He admits to some admiration for his stud son, and also voices the insidious opinion that it’s not really rape if you know, or are involved with, the perpetrator. A chilling detail – that Jack kept his hand pressed over Cara’s mouth throughout the video – belies that view.

McGuinness and in particular Maravan do excellent work to humanise the dialectical disputes, such as protecting your child versus holding them to account. Is it more important to ensure their bright future, one you’ve heavily invested in, or honour your wider social responsibility? But Fritz’s structure – a series of quick snapshots – means key interactions and revelations are sometimes shortchanged or skipped altogether. Excising some of the breathless repetition might allow space for the measured nuance of comparable works like God of Carnage.

Anna Ledwich provides effectively unfussy staging, with Janet Bird’s lighted pixels suggesting an online world reframing the physical one. Anyebe Godwin is appealingly earnest as Jack’s sensitive mate Nick, resistent to the macho lad culture, while Zmitrowicz’s brittle bravado is devastating. The title refers to the length of the video, something that – as Cara points out – will define her forever. What’s a suitable punishment for that? The suggestion of violent retribution is melodramatic, but even then it’s over quickly. Such violation and the critical issues it raises are not so easily dismissed.

• Four Minutes Twelve Seconds at Trafalgar Studios until 5 December 

@mkmswain