I hope you like novelty episodes folks, because ‘Fringe’ has gone for it big time here.

Think the ‘DS9′ ‘Trouble with Tribbles’ call-back, the (awful) ‘Entourage’ making-of documentary or – closest comparison – the black and white ‘X-Files’ Universal horror homage.

In any event it’s difficult for a loyal viewer – already starved of plot advancement – to treat this as other than a tiresome obstacle.

But let’s take some deep breaths and try to be charitable. The high concept here is that Walter gets blitzed on the eponymous high-power cannabis hybrid and is then asked to babysit for the daughter of Olivia’s sister. Come on, you remember her – she nearly had her brain liquefied by Frank Sobotka back in series one.

Anyway, I digress. Walter weaves a story to entertain the child and, as is the nature of these sort of shows, the story gives the actors chance to play their characters in slightly off-beat ways. There’s also a brick-through-a-window subtle allegorical link to the ongoing Peter Bishop trudge of a plot arc.

Walter paints a private eye story with Olivia as a hard-bitten P.I. Boom – obstacle one, as Anna Torv is barely capable of playing Anna Torv. It’s just cruel to ask her to whip up another dimension to the flattest lead role in a TV show since Scott ‘Captain Archer’ Bakula. She is commissioned (in typical convoluted pastiche-Carver style) to investigate the disappearance of a fictional Peter Bishop by a fictional Walter Bishop who, in this telling, invented rainbows and hugs as well as a “glass heart” that Peter stole and that Walter needs in order to survive.

Funnily enough the faintly mawkish fairy-tale element is one of this episode’s strong points. John Noble is great (as ever) as a benevolent mad professor (like a toned down Robin Williams in ‘Toys’) but the crowing glory is the production design, which builds a retro-forties world albeit with hi-tech dressing like lasers and cell-phones. There’s a bit of a musical theme too, with the characters breaking into song on occasion – including a harmonising trio of corpses (don’t ask).

The plot is inoffensive enough, and unravels in fairly predictable but enjoyable way. There are ‘Princess Bride’ style interventions by the infuriatingly precocious child and a head-bangingly obvious metaphor on the stealing of hearts in the context of the Peter/Walter relationship.

There’s also another chance for Lance Reddick to demonstrate why he’s the world’s greatest actor in terms of breathing out slowly and ostentatiously (the long-anticipated Golden Globe for portentous sighing is surely his for the taking).

However, this still feels like a slap to the face for fans of this show. I have already commented on the stringing along of the main plot arc (bear in mind that we the viewers found out about Peter’s origins way back at the end of season one) and this sort of stunt writing doesn’t help.

We get that Walter feels guilty, we understand his selfish justifications for the action he took and we realise that Peter feels betrayed. What we want to see are the results – there’s only so long an audience can be kept hanging on before tension gives way to boredom.

Ironically it’s this season’s other novelty episode (the 80’s flashback of ‘Peter’) that nailed a deliberately tangential setting and tone while also progressing the narrative. ‘Brown Betty’ neither gives us a genuine standalone episode (it is far too closely tied with the ongoing Peter story) nor moves things on any further in a broader sense. No amount of fancy set-dressing or wardrobe work can make up for that.