Lucy Prebble belongs to the posse of scribblers responsible for the HBO hit, Succession. Perhaps in honour of this distinction, her 2012 play, The Effect, has been revived at the National by master-director Jamie Lloyd. The show is a sitcom set in Britain’s most dysfunctional drug-testing facility where two sexy young volunteers, Tristan and Connie, are fed an experimental love potion that may help medics to find a cure for narcissists suffering from depression. Running the experiment are two weird boffins, Professor Brainstorm and Nurse Snooty, who once enjoyed a fling at a conference and whose lust is not entirely extinct. But Nurse Snooty is playing hard to get. ‘Sometimes,’ she tells the Professor, ‘I feel I’m dead but my body hasn’t caught up yet.’ The Professor, a psychiatrist by trade, fails to spot the negative signals here and continues to bombard her with lecherous suggestions.

Meanwhile, in the mixed-sex ward, Tristan and Connie are flirting like mad even though they have nothing in common. She’s a feminist psychology student who likes older professional academics. He’s a penniless half-wit from east London who makes a living by volunteering for medical trials. Yet Connie seems mysteriously smitten with this talentless creep even though he mocks her accent and mannerisms. And she encourages his mistreatment by tittering uncontrollably at his jibes.

To explain her nervous giggles she spouts antique psychological platitudes. ‘Female laughter is a show of submission,’ she says. Since the two lovebirds have swallowed a medical aphrodisiac, their flirtation owes more to pharmacology than to desire and this makes the romance feel contrived and half-cooked.

One night, they break out of the facility and climb the roof of a nearby mental asylum where they continue to flirt by practising ballet twirls and break-dancing. When Nurse Snooty catches them, she delivers a stern warning that sexual congress is strictly forbidden under the terms of their contract. Fair enough, they nod meekly. They promptly dash back to the mixed-sex ward for an eight-hour session of raucous lovemaking which Nurse Snooty and Professor Brainstorm somehow fail to detect. The dottiness of this caper has only just begun. Nurse Snooty takes Connie aside for a girly chat and tells her that placebos are often used in medical trials. Connie gets the hint. Their mutual feelings may not be genuine. Nurse Snooty goes further and tells Connie that she has taken the placebo while Tristan is high on the psychoactive love-potion. Dramatically this makes the story more interesting – but logically it’s senseless. Why would Nurse Snooty endanger the experiment – and her career – by revealing secret data to a volunteer? Answer: the play needs a plot and anything will do. The story grinds towards a lamely predictable conclusion after two long hours.

Jamie Lloyd accompanies the action with non-stop industrial thumping and grinding noises which sound like elevator equipment being tested in a nearby warehouse. The overhead lighting-rig showers the darkened stage with white discs and oblongs of brilliance and although it looks pretty, the colour palette is no different from a zebra, a piano or a nun’s outfit. Every low-budget filmmaker knows this trick: black and white makes boring look classy.

Much praise has been lavished on the performances of Paapa Essiedu (Tristan) and Taylor Russell (Connie) who are commendably fluent and convincing in their roles. But even that harms the show. Watching a pair of loved-up sex-athletes playfully molesting each other for two hours will probably set your teeth on edge.

The Garden of Words is an atmospheric multimedia show based on a Japanese anime. The story is about an introverted teenage schoolboy, Takao, who shelters from the rain in a park where he slowly makes contact with a female teacher, Yukari, who shares his pathological shyness. It doesn’t make for a lively drama. Takao’s ambition is to design ladies’ shoes and after 60 minutes of stage-time he plucks up the courage to take Yakari’s measurements by asking her to cover the sole of her foot with dust and tread on a sheet of A4. This delicately sexy moment is the show’s highlight.

Elsewhere, the production looks like a showcase for experimental effects that compete with each other for dominance. Tinkling piano music, mannered dance moves, poetry flashed up on the rear wall, rice-paper scenery that flaps and dangles in the breeze, and snatches of mime that evoke the hustle and bustle of rush hour in the inner city. It’s less a play and more an arty fairground ride with bells and whistles parping constantly. The show’s outstanding personality is a barking crow represented by a half-bald puppet made of black feathers which flaps and creaks through the action at random moments. Though fitfully beautiful, the show feels like a major slog.