I have a feeling that viewers are now attracted to new comedies in a process similar to ant-colony optimisation. A few try it out and if they like it and return they leave a pheromone trail that attracts others, until eventually there's a swarm clustered round every juicy new episode. Not sure that it's going to happen with Heading Out, Sue Perkins' comedy about a vet plucking up the nerve to tell her parents she's gay, perhaps because it continues to strain credulity with its comic plots.

This week, one running joke concerned her friend Jamie's attempts to become more blokey, an ambition that never remotely threatened to become believable. On the other hand, the surreal moments at which the whole thing turned into a soulful French film, complete with subtitles, were quite funny. The latter seemed true to a state of mind, the former just faked one for the sake of a joke.

Three weeks in and partially closeted lesbian vet Sara (Sue Perkins) isn't making much progress towards the grand parental coming-out inked in for week six. But forget the daft story and relish instead the comedy cameos that light up Sara's world. Tonight Raquel Cassidy puts in a delicious performance as Sabine, Sara's French-speaking ex. Sabine accuses Sara of being emotionally cold, leading to a frank exchange of views that gives us a snatch of cinéma noir-et-blanc.

Sara's very loud, very angry French ex turns up uninvited and colonises the living room where she melodramatically writhes around on the floor. It's a madly over-the-top, heavily accented turn from the estimable Raquel Cassidy (Jack Dee's long-suffering wife in Lead Balloon).

Meanwhile, Sara (Sue Perkins, also the writer) tries to pluck up the courage to ask out the lovely Eve (Shelley Conn). It's fun and sweet-natured and there's great support from Nicola Walker and Dominic Coleman as Sara's friends, dim Justine and precious Jamie.

It's the midpoint of this endearing sex comedy, and romantically inhibited vet Sara (Sue Perkins) still hasn't nerved herself to ask beautiful dog-owner Eve (Shelley Conn) out - and Eve's labrador has finally run out of ailments for her to treat. Even worse, Sara's French ex, Sabine (Raquel Cassidy) has turned up, and is making extremely Gallic scenes (filmed by director Natalie Bailey in moody monochrome). Don't miss Perkins's rendering of the words "susceptibilité puerile!"

Sue Perkins's almost-out gay vet Sara visits chucklesome life coach Toria (The Thick Of It's Joanna Scanlan) in a bid to ease her transition towards revealing all to her parents. She's also busy acting as pet-based marriage counsellor to trophy wife type Julia (Amy Huberman), who is seeking custody of her hound Rufus. Meanwhile, for some vaguely explained reason, she finds herself playing paintball and meeting a Dutch movie star. In summary, the surreal moments are jarring, the narrative is a mess, and then it just ends.

Sue Perkins's gentle comic persona keeps this new sitcom about a slightly useless vet ticking over. Sara (Perkins) is a lesbian who has yet to come out to her parents, but is being chatted up by one of her clients. Tonight she tries to help a woman retain her dog during a messy divorce, and there's an amusing scene when she visits Toria, her life coach - played by the endlessly funny Joanna Scanlan from The Thick of It and Getting On.

In conversation with Dominic Coleman

Dominic Coleman talks about his role in Heading Out...

New to BBC Two, Sue Perkins stars in a new sitcom about a lesbian vet - although she herself has described it as not being a "gay sitcom".

Perkins plays Sara, who in the opening episode celebrates her 40th birthday. However, she has one major problem coming up: she's never told her parents that she's gay, making up bizarre-sounding boyfriends like a Frenchman who sells false legs. To make things worse, they're coming up to see her in a few weeks. As a result, for her birthday her parents decide to hire Sara a rather unorthodox (and to Sara an annoying) lifestyle coach called Toria (Joanna Scanlan), to give her the courage to finally come out. If Sara fails to do so, Toria's under instructions to tell Sara's parents herself.

This opening episode was very good. The first scene, in which Sara deals with a cat called Mosley owned by someone who seems to be keen on alternative therapies (Ella Kenion), is great. It gets better when she starts to put the cat down, only for the owner to change her mind half-way through. This leads to an even better scene starring Mark Heap as the undertaker at a pet crematorium, in a typically bonkers role that we are used to seeing him in. The laughs keep coming.

Much of the better comic moments are slightly skewed. It's not off-the-wall surrealism, it's just slightly odd, but in this case odd works well. Whether it's a scene involving a netball team doing a haka or the idea of a restaurant which tells you the name of the cow you are eating, it all seems to be working well.

And Sue's right - the fact the lead character's gay appears to be something just in the background. This series has potential, but the big test is still to come.

Heading Out (Tuesday, BBC Two), written by and starring Sue Perkins, was as delightful as it was unexpected. The Sue Perkins persona that is familiar from comedy quiz shows is sharp and confident, whereas the character she plays in this - a vet specialising in cats and dogs who is hopeless at making eye contact, and finds it impossible to come out to her domineering mother (Harriet Walter) - is shy, awkward and wistful. "I feel shame all the time," she said on the subject of being in the closet. "It's like ivy creeping round me."

There were funny lines, too: "People think vets spend all their time with their fist up an arse, but that's only 95 per cent of the job." And to her girlfriend: "Remember that night you made me go cross-eyed? I'm quite keen to repeat that." But mostly this was the comedy of embarrassment and self-deprecation.

Most successful British sitcoms are variations on the same few recurring themes, such as generational conflict (Steptoe and Son, Ab Fab), the cunning underdog taking on the world (Porridge, Blackadder), or dysfunctional friendship (Peep Show, The Likely Lads), but this theme, coming out to your parents, seemed fresh to me.

In episode one, her straight best friend (a nice twist on the Gay Best Friend you always find in romcoms) attempted an intervention, to chivvy her along. Whether this conceit can be sustained for five more episodes remains to be seen, but already I like the characters enough to want to find out.

Review: Heading Out - Pets, eh? They're enough to drink

Fortunately, Sue Perkins, she of the endless witty one-liners on The Great British Bake Off, not only stars as Sara, but also writes, which elevates Heading Out to watchable status. OK, it isn't about to blow anyone's mind, but it's blessed with beautifully written, unexpected lines and Perkins's ability to be droll and vulnerable at the same time.