Last Night's TV: Beckii: Schoolgirl Superstar at 14, BBC3<br />Mistresses, BBC1<br />Natural World, BBC2

It isn't clear what, exactly, about Rebecca Flint's dancing has propelled her to international stardom. Certainly not to British viewers over the age of 16, anyway. But propel it has. Beckii (or "Beckii Cruel", as her pop alias would have her) is properly, amazingly famous – much more so than many of the Identikit faces gracing the pages of Heat. In Japan – the world's 10th most populous country – Beckii is a superstar. Her personal website is the third most visited music site in the country, and when she makes the trip over, she struggles to walk down the street without being accosted by gangs of giggling teenagers wanting a photo.

That first fateful YouTube video – shown last night on Beckii: Schoolgirl Superstar at 14 – is innocuous enough. A teenage schoolgirl dressed in vaguely punky high-street attire, inspired by her love of Japanese manga cartoons, dances to a catchy pop tune with the chorus "danjo, danjo, d-danjo". She looks slightly awkward, not like someone who would get very far in Britain's Got Talent, but then what do I know? The video has amassed millions of hits. Almost overnight, her innocent hobby of dancing in her bedroom turned Beckii into a celebrity.

Her parents appeared unaware of their daughter's activity until an email arrived from a Japanese music manager. He wanted to take her to the country and turn her into a pop star. Her bambyish, slightly wan look is, apparently, le dernier cri in Japan. Audiences find her "unbelievably cute". Where some might have refused point blank, Beckii's parents allowed her to take the chance (not without a hefty dose of scepticism from her copper dad) and – judging from her experience, at least– it doesn't appear to have been a bad decision. When she wasn't at school working on her GCSEs, she was off in Japan, dancing for 16 million on live television and putting in appearances at the British Embassy. She didn't find it difficult, she said, because she felt in tune with Japanese culture. It helped, no doubt, that she is an exceptionally grounded young girl, and remarkably intelligent for her age.

Of course, it wasn't all stardom and celebration. Inevitably, Beckii's exotic success brought taunting at school, where her classmates would occasionally bust into renditions of "Danjo". Online, she has "haters", she explained: people post pictures of her face distorted and unrecognisable. She has also struggled to make much money. Fame, particularly of the internet-generated variety, doesn't necessarily equal riches. And then there has been the worrying sexual element of her exposure. Parents and manager both expressed eagerness to move away from it, though it's difficult to see how. When Beckii got her own girl band, one bandmate's YouTube statistics revealed middle-aged men to be the highest viewing demographic. Perhaps it is just as well, then, that by the end of the programme, Beckii had decided to keep her education as her principal focus.

Well, that didn't last. Mistresses? Chaste? They're all having it off now. Trudi's gone from Nigella crossed with The Apprentice, to Nigella crossed with certain viewers' imaginations, flirting with her boss, kissing him on the sofa. Her husband, meanwhile, was behaving just as badly – worse, possibly, given the object of his affections is Katie, Trudi's BFF and doctor-in-exile. Katie had decided to give internet dating a go on the advice of her mother, the eternally lovely Joanna Lumley. Even Sensible Siobhan was giving it a go, trying to snog her former flame the day before his wedding (he deserved it, the rascal, marrying the youthful, leggy Alice like that).

The only one who wasn't behaving badly, in fact, was Jessica. This can't last, can it? Judging by the flash-forward to next week episode: no. In the meantime, I'm just glad we've got the old Mistresses back.

Natural World gave us another stellar piece of documentary last night, trailing a mother sea otter who had decided to leave the kelp forests in order to raise her pup in the luxury resort of Monterey, California. It was a weird – if rather tasteful – choice. Billionaire yacht owners don't tend to take kindly to otters bashing crab shells against their beloved investments; neither are there many crabs to be cracked open. Still, our heroine did OK, sneakily sussing out the surroundings before causing too much criminal damage (I mean this quite literally; she was quiet extraordinarily cunning) and happening upon the parking bay of Jim, a quiet, respectful businessman.

Jim barely seemed able to believe his luck at her choice and, to be honest, I don't blame him. Sea otters – and this is the zoologist in me speaking, strictly scientific objectivity and all – are unbelievably, incredibly, reduce-to-baby-talk-ingly adorable. Seriously. The cutest thing you're likely to see on TV – at this time of night anyway. And they are quite staggeringly doting parents, a fact that only adds to their general fluffy appeal. They cuddle, constantly, rolling around together in maternal embrace. Every day, mum and pup would head out, her carrying him on her tummy while she backstroked around the marina, looking for a sunny bit of deck to park him on. Then she would dump him – for as long as she could without him crying – and hunt for fish. Every so often she would return, lulling her pup back to sleep in the water before plopping him down in the sunshine again. It was utterly, utterly enchanting.