This is “plop opera”. Underground, slime is sexy and cleanliness next to abomination, and - not much happens for an act. To compensate, act two bundles together Fungus’ midlife crisis and son Mould joining yoof rebels fired up on video nicies in a “clean is cool” kick.

Ken Campbell directs, so the action - when it arrives - fizzes along inventively. And Claire Lyth’s designs take Briggs’ illustrations successfully from page to stage in a riot of gungy green caverns and rows of neat human residencies. Mac MacDonald is an affecting Fungus, Tracy Harper full of juvenile zest as Mould. But with vapid, predictable lyrics and the kind of music that gives speech a good name the material defeats them. This is flop opera, drear.

Until June 3. Tickets: 01203 553055.