There are no easy answers in this play set in Nazi-occupied Guernsey, both in terms of plot and moral viewpoint. The ambiguity of Moira Buffini's script is symbolised by the appearance of Gabriel himself, a character whose name may not even be Gabriel and who cannot remember anything about his own identity.

He has washed up on the Guernsey coast and is in equally fluent in English and German. Who is he? Estelle, the young girl whose brother, a fighter pilot, is missing in action, chooses to believe he's a sort of real-life angel.

Von Pfunz, the complex Nazi brilliantly rendered by Paul McGann (Withnail and I, Doctor Who), is convinced that Gabriel's mastery of the German language means he is a Nazi – he just doesn't know it. This confusion plays out entirely in a small Guernsey house, where Jeanne Becquet hosts her daughter Estelle (Venice Van Someren) and daughter-in-law Lily (Sarah Schoenbeck).

In a moving scene early in the play, Estelle and Lily embrace the unconscious Gabriel after dragging him out from the cold having found him on the beach. Gabriel becomes a surrogate for Miles, the brother and husband lost to the two women. Von Pfunz occupies the family's house, and McGann exquisitely portrays the tortured soul of a soldier, a poet desperate for the truth – whatever that means in World War Two.

Some of Von Pfunz's scenes with Jeanne (Belinda Lang) are electrifying; the Nazi commander flits between menacing and philosophical, while Jeanne, whose husband died decades ago, is responsible for some of the best lines in the play.

"I'm not afraid of my cruelty...only my cowardice" is an instant stand out. Director Kate McGregor does a terrific job of gradually building up the tension of the play while at the same time making each character ever more complex and unknowable.

Von Pfunz's quest for the truth is ultimately denied, but the play is all the better for its refusal to grant closure to the audience. Theatregoers spill out into the night wondering about the true identity of Gabriel, the history of Von Pfunz and the precarious future of the vulnerable but dogged Guernsey family. Deliciously dark questions to ponder.

As a postscript (and regular Sussex theatregoers won't be surprised to hear this) but Eastbourne Theatres are doing a splendid job of making the theatre experience immersive and inclusive, from the friendly staff to the packed and informative programme (with theatre-based trivia and quizzes). A real treat.