“See you.” There’s a casual remark for you, a throwaway line that you never pick up on because there’s no point: of course you’ll see each other sometime. But what happens when it’s not true – when you don’t see your son again? This is the question asked in I’m Missing You, which tells the story of Sam, whose son Ian is missing.

Over 200,000 individuals are reported missing in the UK every year, but around 99% of all cases are solved within one year. Ian is in the 1% that are not. At the time of his disappearance he was 18 years old, which is important, because it means that the police won’t pass on his whereabouts without his permission. Ian was last seen at a Tube station on the Northern Line and it’s down on the platform that I’m Missing You begins. It’s where it ends too, twenty years later.

It’s the questions that won’t go away that hurt the most: there’s “Why?”, of course, and “Wasn’t I a good parent?”, and the desperate plea of Ian’s little sister to her parents – “Why don’t you see me [anymore]?” The questions go unanswered and break the family, but Sam comes back to that platform day after day, year after year. He becomes a fixture, known to the station staff and social services. It works as a two-hander, moving from one related encounter to the next. It’s slightly forced, obviously compressed, and it is set in the quietest Tube station with the most well-spoken staff that I’ve ever chanced upon.

Codge Crawford as Sam summons all the pathos that a father in this situation deserves. I thought it was an outstanding performance, conferring dignity upon a man whose situation appears hopeless and wretched. “Real theatre about real people” is what writer and performer Helen Fox wants to achieve and – for good measure – Crawford puts in a second role as Andy, station manager and part-time driver. If first his anguish and later his kindness as Ian don’t get to you, then Andy’s soft Geordie tones will.

The passage of 25 years is neatly signalled by the flyers and signature tunes of the decades; and the use of Adele’s Hello is a knockout, laying low any reservations you might have about the credibility of what you’ve just seen.