With Euro 2012 over and the Olympics not yet started the range of get-out-of-jail cards boyfriends can play in their attempts to avoid seeing this movie have been drastically reduced. Those who are marched to the cinema, however, may be pleasantly surprised by Part of Me, an engaging portrait of a mega-selling misfit who has kept a firm grip on reality while living a Wonkaesque public life. For Perry's fans, the documentary doubles up as both backstage pass and best seat in the house, with the monotony of one concrete changing room after another offset by the explosion of colour and good vibes which awaits on the other side of the curtain. Both Katy Cats and, well, scaredy-cats will end up having a higher opinion of Perry at the end of Part of Me than they did at the start.

If ever an artist sounded like a one hit wonder, it was Perry in the I Kissed a Girl Summer of 2008, but, as directors Cutforth and Lipsitz' film shows, there was plenty of steel behind the sass. A failed gospel album as a teenager and the experience of being dropped by Island/Def Jam before EMI took a punt meant that Perry arrived on the charts with the thick skin of a music industry veteran and a determination to stick around for as long as possible. She has gone on to become the only female artist in history - Michael Jackson being the only male - to have five US Number One singles from the same album. As usual, it's the struggle not the success that makes for the most interesting viewing and the archive footage - Perry in gospel and singer-songwriter mode, scrapped studio sessions - and testimonies from family and colleagues return Perry to the status of underdog while also highlighting just how much she has evolved as a performer.

Co-produced by Perry herself, it's hardly surprising that Part of Me puts her in a good light, but the offstage time captured here does not feel like a performance for the cameras; Perry proves that she is someone with a real affection for her fans and who isn't running for the hand gel after meeting them. Along with providing them with salvation singalongs, they see Perry as an oddball who has managed to find her way in the world, with her outsider status as the child of Pentecostal preacher parents re-enforcing the belief that their dreams can come true too. The story of that upbringing deserved extra screen time, as did Perry's parents, who come across as being proud of their daughter while also struggling to make sense of the life she has made for herself. Mom's response when she's asked what she thinks of I Kissed a Girl is of the gritted teeth variety - more of that would've made for a better, more rounded movie.

Yes, yes, you say, but what about Russell Brand? Cutforth and Lipsitz were in the odd position of starting their film with husband and wife very much in love (they did look right together) and having gone their separate ways by the end of it. Classily, there is no mud-slinging here (no interviews with Brand either, in case you were expecting them), but the trauma of the split after Perry's efforts to keep the relationship alive long-distance is on screen. Those of us whose default mode for assessing celebrity unions is blunt force cynicism will be tested here. It comes across that Perry really loved Brand, and you end up feeling more sorry for her than she feels for herself. She talks eloquently but far too briefly of the challenges of trying to progress a career and a relationship, with quotes of the quality of "a baby can't have a baby, and I'm still a baby" proving that the film needed more face-to-face time with the singer.

Despite the heartbreak, the show goes on - through tears it must be said. Just as the punters are sent home from the gigs with a feeling of uplift, so too will they be from the movie. "I feel like I've got a second chance at a brilliant life," says Perry. "Mind you," she concludes, "I already had a brilliant life." There's someone who deserves whatever happiness awaits her - would that we could all keep such perspective on our own paths.

Harry Guerin