3 stars

It's hard to believe it's been six years since Elsa, Anna and friends took the animated world by snowstorm, raking in $1.9 billion for Disney and making Frozen the most successful animated film of all time. No surprises, then, that there's a sequel, and Frozen II is an upbeat, energetic and safe follow-up, unlikely to upset the huge fan base with the return of its talented voice cast, easy-to-remember songs and never-a-dull-moment storytelling.

One of the secrets of Frozen's success is its focus on sisters, an oddly overlooked relationship in the history of animated film. You may recall at the end of the original movie, the two sisters - who have been at odds for most of the story - are reunited in both love and a determination to never again let their little Kingdom of Arendelle fall prey to evil.

Frozen II opens with a flashback: Anna (voice of Kristen Bell) and Elsa (Idina Menzel) are young girls listening to a story told by their parents of the land of Northuldra, home of the Enchanted Forest. They discover that it is also the place where their grandfather, King Runead, clashed with the local people, causing the spirits of the land to seal the enchanted woods in mist and fog, along with all those inside.

Flash forward and Elsa starts hearing a voice calling her northwards. At first she refuses the call, but when Arendelle is ravaged by the unhappy spirits of the land, she realises she must journey north and face the unknown.

Of course, she's not going to have this adventure alone. Anna insists on coming, along with her clumsy but lovable boyfriend Kristoff, (Jonathan Groff) his reindeer Sven, and the sentient snowman Olaf (Josh Gad). With their kingdom in peril, they head off to restore the balance of nature, find the truth about the past, and learn more about themselves. As Olaf suggests (without knowing what it means): this is a story of "transformation".

While there's a warm predictability to the adventure, screenwriter (and co-director) Jennifer Lee develops some nifty devices for the animated world of snow and ice. The best of these is the beautiful idea that water has a memory, and that in the north there's a river where memories can be found.

Elsa discovers that she can use her powers to reveal frozen moments from the past: some that surprise, others that are deeply emotional. Go too deep into this river of memory and you can drown: that's the distinctly Jungian warning.

Lee and co-director Chris Buck keep the focus on the two sisters, Elsa thoughtful and troubled by what her powers might mean, and Anna working out how to become braver.

Kristoff and his romance with Anna is carefully pushed to the background and he, along with Sven and Olaf are used mainly for comic relief.

I have to admit that I wasn't a fan of the snowman character in the original film, but the children around me in the screening I attended were clearly huge fans of his antics, laughing hysterically during his solo musical number when he sings When I'm Older as he deals with the playfully pesky powers of earth, air, fire and water. He may look out of place, but Olaf provides the classic clown voice: explaining when the plot needs clarification, mocking when everyone gets too serious, and offering the occasional very adult aside.

Multi-award-winning musicians Kristen Anderson Lopez and Robert Lopez return with all the songs, and while the big number Into The Unknown is no match for the first film's Let It Go, all the songs resonate emotionally, especially Evan Rachel Wood's tender lullaby All is Found which opens the story.

It will undoubtedly be a huge hit with its followers (and their parents), offering some good advice about finding yourself, dealing with the unknown and doing the next right thing.

Elsa and Anna are two versions of the modern princess, both of them very capable, and very watchable.