Luna Gale, Hampstead Theatre

Can we really distinguish between experience-based judgement and personal bias? Caroline, the social worker at the centre of American writer Rebecca Gilman’s latest "issue" play, trusts a gut instinct informed by her 25-year career, but those decisions – which shape the lives of her young charges and their families – are gradually revealed to be subjective in the extreme.

It’s a refreshing subversion of the traditional "caring people versus intractable system" narrative. The bureaucracy of the Iowa Department of Human Services is unwieldy, but navigable – and vulnerable to manipulation. When Caroline (Sharon Small, pictured below with Corey Johnson and Caroline Faber) meets meth-smoking teenagers Karlie (Rachel Redford) and Peter (Alexander Arnold), who brought severely dehydrated infant daughter Luna to the emergency room, it’s an initially easy decision to remove her from unfit parents and place her with grandmother Cindy (Faber). But Cindy, who decides to seek permanent custody, is an evangelical Christian, in thrall to her pastor (Johnson) and proclaiming the end of days; Karlie is desperate to save Luna from the environment she escaped. When Caroline’s involvement with the case becomes deeply personal, she decides to rig the system.

Her decision is played as a gasp-inducing cliffhanger, a melodramatic twist that disrupts Gilman’s socially conscious naturalism. This and several subsequent soap operatic beats are retroactively tempered or explained, but it doesn’t stop them feeling out of character and tonally jarring in the moment. The starkly schematic, suspense-driven plotting distances us from the emotive subject matter, while also limiting the possibilities for both challenging, nuanced debate and richly metaphorical theatre. A signposted subplot adds to the overload.

Yet there are smart, topical observations about the devastating effects of slashing welfare budgets: Karlie’s recovery is hampered by long rehab waiting lists, and the alternative, wittily named support group "Mothers Off Meth" AKA "MOM", offers nothing but craft nights. Officious, state-appointed boss Cliff (Ed Hughes) has a beady eye on cost-cutting, and, following numerous redundancies, even devoted Caroline longs for early retirement to escape an avalanche of cases – although, with just two mentioned in the play, the latter point is only really illustrated by Lucy Osborne’s striking set, framed by towering shelves of box files.

Michael Attenborough, returning to the venue he ran in the late 1980s, draws engaging performances, albeit with wandering accents. Small’s compelling workaholic is both benevolent and crippled by her own damage, Faber and Johnson bring sincerity to potential caricatures, and Arnold’s Peter grapples believably with the complexities of adulthood. In a world full of people defined by past abuse, seeking miracle cures through faith, drugs or redemptive work, his growth is laudable, even as we face the fact that the systems designed to help emphatically failed him. It's a thought-provoking discussion of parental, individual and public responsibility, but too bluntly didactic to truly move.

• Luna Gale is at Hampstead Theatre until 18 July