A week before Valentine's Day and anyone who's lost faith in the power of everlasting true love should be sent on a blind date to Abi Morgan's new play.

A collaboration with director/choreographers Scott Graham and Steven Hoggett's Frantic Assembly company, it charts four decades of marriage between Maggie and Billy via two sets of actors. It cuts through the hearts and flowers to get to the real-politik of a relationship that is a mirror for some, and an education for others. Mid-life crises, affairs and seven-year itches are all intact.

Morgan's ongoing fascination with the ageing process follows both her recent National Theatre of Scotland play, 27, and her script about another Maggie for the film The Iron Lady. Yet in Frantic Assembly's head, hands and feet, the company's trademark physical tics elevate her words to somewhere else again. As back-dropped here by Merle Hensel's stately design, Ian William Galloway and Adam Young's broody video projections and especially Carolyn Downing's sound design, here the play's execution feels softer and less pumped up than Frantic's usual fare. At times it's almost too quiet. If the younger members of the cast need to project more effectively, the gymnastic interplay between the generations is exquisitely realised.

One gorgeously wordless moment captures the play's heart, when Edward Bennett and a magnificent Sian Phillips as the older Billy and Maggie reach out for their younger selves, played by Sam Cox and Leanne Rowe. Even then, it seems, there's recognition that a time will come when they'll both have to let go. As the title suggests, Lovesong is a beautifully fragile elegy that's to die for.

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