The depths of Wastwater in the Lake District are a metaphor of hidden secrets and emotional undercurrents in Simon Stephens’ enthralling new triptych of inter-linked plays, directed with calm, unshowy assurance by Katie Mitchell, and beautifully acted.

The common theme is that of child care in the community: a foster mother (Linda Bassett) says goodbye to her troubled charge, Harry (Tom Sturridge), who is catching a bus to take a flight to Vancouver.

Then an art teacher, Mark (Paul Ready) and a married off-duty policewoman in child protection (Jo McInnes), with a steamy past in pornography, prepare to have sex in a hotel bedroom; another teacher, Jonathan (Angus Wright), is quizzed by a vile child trafficking agent, Sian (Amanda Hale), before meeting his little Filipino purchase.

The latter case is only one of adoption, not sexual kidnap, or at least we hope it is. Nothing is quite right between any of these characters, but this uncertainty is all part of the texture of their relationships, which are part mercenary, part provisional, all in transit. Suitably enough, each play takes place near Heathrow, and dialogue is punctuated by low rumbles and loud take-offs.

Mark taught Harry’s best friend who died in a car crash. Sian is another of Frieda’s foster children. But the closeness of the plays only underlines the distance between people, and this melancholy of separation is vividly expressed in Lizzie Clachan’s superb design: an abandoned conservatory, then a large impersonal bedroom, finally an echoing brick warehouse with steel girders.

With the distance goes the violence: accidental violence in the first play, a masochistic undertow in the second – Jo McInnes is literally panting with expectation throughout before demanding to be hit (well, Stephens does have to keep an eye on his German market) – and intellectual bullying and taunting by the agent in the third.

The plays exercise an insidious fascination and touch many an exposed nerve in our views about how we look out for our children and what we should expect (or not expect) of those charged with caring for them in public. Much of the evening is shocking, even upsetting. But it’s seriously written and seriously enjoyable.