IT’S the sing-along phenomenon that has sent a giddy chill down the world’s nape. Originally planned as a modest Disney musical, Frozen has become a global juggernaut, reaping billions in merchandising revenue and whipping fans, young and old, into (usually) good-natured hysterics.

But a paucity of Frozen toys prompted an angry face-off among parents at a north Dublin store and such incidents have have been reprised globally: children have a bottomless appetite for Frozen ‘bling’ and their mothers and fathers will do whatever to make their wishes come through.

At first pass, the popularity of Frozen is hard to fathom. Why has it touched a chord with audiences, where so many Disney animations have not?

We must consider the film’s unusual gestation. Originally, the plan was for a straightforward adaptation of Hans Christian Andersen’s The Snow Queen, which tells of two orphaned princesses, one pure of heart, the other jealous and spiteful.

But things went off-script, when Disney hired Kristen Anderson-Lopez and Robert Lopez, the husband-and-wife team behind the Broadway hits, Avenue Q and Book of Mormon, to write the songs for Frozen. First, they penned ‘Let It Go’, a ballad to be performed by ‘evil’ sister, Elsa. However, in the hands of the Lopez spouses, Elsa was transformed from cartoon villain to someone with whom you couldn’t help but empathise. Uplifting and strident, ‘Let It Go’ was not the kind of ballad you expected a vile sibling to sing — it was about learning to love yourself, to celebrate yourself, flaws and all. “A [song] that said, ‘Screw fear and shame, be yourself, be powerful,” as Anderson-Lopez said.

Disney executives were stunned when they heard ‘Let It Go’ and understood they would have to rethink the movie — to present Elsa and Anna not as adversaries, but as sisters, full of love for one another despite their differences.

After decades of cardboard cut-out heroes and heroines, Frozen thus became something different: a fairytale grounded in the real world. And so, almost by accident, a phenomenon was born.

Initially, however, it seemed that Frozen would be little beyond a cult hit. Released in November, 2013, reviews were lukewarm: critics said Frozen was competently assembled, yet hardly likely to set the world ablaze. How wrong they were: Frozen overtook Toy Story 3 to become the most lucrative animation of all time, with box-office receipts in excess of $1bn (and with the DVD selling 3.2m units on the day of its release).

What sets Frozen apart is that it is the first hit children’s movie of the social-media era. Soon, fans were exerting ownership over ‘their’ film — and over ‘Let It Go’ in particular. Initially, Disney was minded to stifle the song by making it unavailable on YouTube. However, Disney soon came to appreciate that the internet was driving the popularity of Frozen rather than eating into the profits of the ‘House Of Mouse’.

Cover versions of ‘Let It Go’ proliferated. The song had taken on a multitude of meanings: on YouTube, you could clap along to versions by toddlers, African tribesmen, cancer survivors, gay couples. It had become nothing less than a global anthem of self-empowerment.

“At night, my daughter and I go online and see what’s new,” the director of Frozen, Jennifer Lee, said recently. “One girl did an ‘autism version’ of ‘Let It Go’, about accepting that the thing she thinks is the most negative about herself is actually a positive. That really moved me.

“I’ve also seen people making costumes, reshooting scenes, and the whole movie acted out by kittens. I love the fact that the fans can make it their own and that they’re moved to spend time creating something.”

There is no, one, simple explanation for the movie’s impact. Children — little girls especially — love it because it presents strong female characters unencumbered by the need to wait around for a prince to save the day (these feisty young women are more than capable of saving themselves) .

For older audience members, the film teaches us that happiness comes through self-acceptance — if we learn to love who we are, all the rest will fall into place. In an age when social media and cyber-bullying can corrode our self-belief, that’s an enormously powerful message — like Disney, we may be astonished by Frozen’s success, but it is hard to disapprove of the movie and its uplifting message.