Three stars

There's no denying Taika Waititi's spectacularly irreverent sense of humour. If you have seen Boy (2010), What We Do in the Shadows (2014) or Hunt for the Wilderpeople (2016), you'll know that the New Zealand writer-director is a fan of the quirky underdog and the awkward outcast, and always likes his humour unpredictable and unsentimental. In Jojo Rabbit he turns his wicked attention to Adolf Hitler's ideological grip on Germany towards the end of the war, as seen through the eyes of 10-year-old Johannes Betzler (Roman Griffith Davis), known to his friends as "Jojo". He's our underdog hero: scrawny and sensitive in a time of wolves, and trying hard to be a good little Nazi.

Jojo lives with his busy but devoted mother Rosie (Scarlett Johansson) and a memory of his father, who's away serving in the army. Also missing from his life is an older sister Nina, who has died of influenza. Alone in a big house, we can easily forgive Jojo for having an imaginary friend. But, surprise! The friend is none other than Adolf Hitler himself, played by Waititi as an infantilised version of the ruthless dictator. Together, Jojo and "friend" get through the torments and bullying that come with being unpopular at a Hitler Youth camp weekend. It's run by one-eyed war veteran Captain Klenzendorf (Sam Rockwell) and his fanatical sidekick Fraulein Rahm (Rebel Wilson) with the young boys learning to throw grenades and burn books. After his return from camp - where Jojo gets his "rabbit" nickname - the main story kicks in when our young hero discovers that his mother is hiding a Jewish teenager, Elsa (Thomasin McKenzie), in the attic. Torn between the moral influences of his imaginary friend and those of his mother, Jojo is encouraged to study Elsa and learn about her Jewish ways, a strange project that brings them closer together.

There's no doubt this is a crowd-pleasing romp, poking fun at the everyday tragedy of a campy, stereotypical idea of Nazism. Hitler Youth leaders, Gestapo Agents and Nazi soldiers are portrayed through extreme parody, much as you'd find in Hogan's Heroes or Mel Brooks' The Producers - except that Waititi adds a juvenile delight to proceedings. It's hilarious at the start, best executed in the hands of gifted actors like Rockwell and Wilson, but definitely wears thin as the movie proceeds. Fortunately, the developing relationship between Jojo and Elsa provides the beating heart of the story, with Griffith Davis delivering a nuanced performance as Jojo comes of age, marked by the unravelling of his indoctrination. McKenzie plays the teen muse with tenderness but there's an unlikely knowing to her performance, perhaps needed to bring a sense of hope to the desperate situation, but more probably the side effect of Waititi's pantomime approach to storytelling. "You're not a Nazi, Jojo," Elsa says to him. "You're a 10-year-old kid, who likes dressing up in a funny uniform and wants to be part of a club." It's spoken like a wise woman from the future, and clearly more for the audience than Jojo.

Waititi, who claims Jewish ancestry from a grandmother, had no intention of playing Hitler himself when he first wrote the screenplay, but was encouraged by his producers to take the part. (He's admitted in interviews that it would have been a risky casting decision for any well-known actor). It's an insanely absurd performance, with more than the occasional nod to Chaplin's work in The Great Dictator, but Waititi can only deliver a cheeky one-note tune that gets a little repetitive.

The story is inspired by Christine Leunens' novel Caging Skies, and Waititi has turned a sombre and serious tale into goofy entertainment, the sheer stupidity of it all easily mistaken for insensitivity. But the quality of the cast - especially Johannson in truly excellent form as Jojo's mysterious and playful mother - and the inspired music choices, make for easy viewing. Pack up your political correctness, hold onto your jaw and enjoy the ride.