Frank Martin’s 1941 oratorio profane, Le Vin herbé, is a recounting of Tristan and Iseult’s love-death far removed from the shattering excess of Wagner’s music drama. Intimate and understated, with a darkly ravishing chamber score, in director Polly Graham’s revelatory staging for Welsh National Opera, it proves loaded with existential dilemmas as potent as the philtre the lovers fatefully imbibe.

Caught between overwhelming passion and chivalric duty, Tristan and Iseult – sublimely sung by Tom Randle and Caitlin Hulcup – can neither live nor die without the other. Their struggle is at once liminal and physical; grounded in designer April Dalton’s matt-black innards of a set, it and they are exposed in lighting now starkly monochrome, now lustrous with sudden colour.

The WNO strings-piano octet, with conductor James Southall, are placed onstage before a raised (ship’s) walkway as the pulsating, almost liturgical, heart of the production. Around them according to Jo Fong’s fluid movement design, and addressing us directly, the WNO chorus alternately surge or hold position; both witness to and narrator of an unfolding tragedy with which they intently interact. Solo storytellers step forth alongside King Mark (Howard Kirk), Brangien (Rosie Hay) and more. All were superb in a vividly poignant marriage of music and theatre.