Even with songs and music to soften the plot, The Unlikely Pilgrimage of Harold Fry still has the impact to shock as well as grab the heartstrings after a letter from a dying woman changes the life of an unhappy middle-aged man trapped in a sterile marriage, writes Sheila Checkley.

Mark Addy brilliantly portrays the gentle Harold sharing his empty life with his embittered wife Maureen (a moving performance by Jenna Russell), frightened to leave the house.

A letter from a Queenie Hennessy (versatile Amy Booth-Steel) arrives at Harold’s Devon home, telling him she is dying in a northern hospice and wants to say goodbye. Harold vaguely remembers her as a woman he gave lifts to when they worked in a brewery.

His brief letter of reply is set to change his life as well as unlock tragic memories, after instead of “popping out” to post it he keeps walking on a 500-mile pilgrimage to see Queenie before she dies.

On his travels a char lady with a medical degree tends his sore feet, and formidable garage girl Kate (memorable Sharon Rose) inspires him to keep walking while performing a dazzling dance sequence.

As Harold gains media fame hippy pilgrims want to walk with him, but their exuberance almost brings him to a halt, and he adopts a stray dog, masterfully animated by Tim Tatzber, which almost steals the show.

While walking Harold conjures up images of his student son David (Jack Wolfe, a riveting acting and musical performance), remembering the gulf that grew between them.

The final scenes, with clever direction by Katy Rudd, dramatically reveal the tragedies that almost destroyed Harold’s marriage but also the debt he owes to the dying Queenie.

The Unlikely Pilgrimage of Harold Fry is being staged at Chichester Festival Theatre until June 14.