Puppets and a children's story – two of my favourite things – what could be better? I was nervous, though, heading off to Northampton. Nothing to do with the partnership of producing companies: Royal & Derngate is a regional theatre; Polka specialises in theatre for children and young people; Little Angel in puppet pieces for all ages. All three have strong reputations. But… the Moomins… on stage?

It's 100 years since the birth of the Finnish writer and artist Tove Jansson, who created Moominvalley and all its inhabitants. Her simple line drawings of strange-shaped creatures and the stories of their adventures seem to live in a world that nestles in among the atoms of our everyday realities. It's hard to think of them as "imaginary". The thought of seeing made-up versions of them in solid form made me feel a bit weird.

Ben Glasstone's music, combined with Laura McEwen's design and Will Evans's lighting create a space where the air is denser than normal and tinged with colour. Small shifts in a seemingly simple set transform kitchen to flooded forest to floating theatre with swift ease. Here, the 11 knee-high and smaller rod puppets that are the Moomin family and their friends (as well as the life-sized, less-than-friendly Hemulen Policeman) breathe and move. Designed by Lyndie Wright, they are operated with skill and affection by Nick Ash, Ruth Calkin, Claire Harvey and Lori Hopkins, who are always visible but never obvious. Phil Porter's adaptation is excellent within its constraints of simplification. Returning from Northampton, I was full of my visit to Moominvalley.