A gripping thriller, it revolves around that staple of so much drama, the love triangle. Mackintosh and Nardini play Tom and Lois, a pair of married police officers, whose already rocky marriage is put under greater strain when he goes undercover to act as a pimp. He soon gets rather too immersed in the seamier side of London life.
As the crew restored a former massage parlour in the East End, the depiction certainly seemed authentic. "There was a constant stream of men knocking on the door saying, `Oh you're open again, fantastic'," recalls Jane Fallon, the producer of Undercover Heart. "At one stage, two props men got arrested by the police while they were decorating it. We couldn't fault the accuracy of their work, that's for sure."
When Tom vanishes, Lois finds herself attracted to his best friend, Matt (Lennie James), another cop, who helps her search for him. "The basic premise of Undercover Heart is that when the man goes missing, the best friend and wife fall in love while they try to find him," says Bowker. "They wonder whether it's a good idea to find him at all. So it's a kind of morality tale. Sex, love and guilt struck me as fertile ground."
I'll say. Works as diverse as Our Mutual Friend, by Charles Dickens, and Harold Pinter's Betrayal have profitably mined such three-cornered affairs. There is rich, inherent, dramatic friction in the set-up.
"Undercover Heart is a grown-up story about betrayal and love," James reflects. "Matt is a decent bloke who does an indecent thing. It is about what happens if you are not a bad person, but you do actually step over the line. Most people who have affairs are decent people; the problem is, they can't retract."
In Nardini's view, "the series is very dark. You get right into the core of these people. It looks at a dysfunctional marriage and why people have affairs. Steven Mackintosh's character, Tom, is so obsessed with work, he's on the verge of losing his sense of self. When he gets close to a victim in a ghastly case, Lois feels a sense of betrayal which pushes her towards his best friend. Then she starts to have a great time," she adds with a throaty laugh.
Being grown-up, the series is not inhibited about what affairs inevitably involve: sex. But, Fallon contends, the bedroom scenes in Undercover Heart are in no way gratuitous. "There is quite a lot of sex, but not long, lingering shots of people's bodies. It's not an exploitative show."
Nardini, whose recent on-screen exploits have caused much salivating in the tabloids, agrees. "I never felt exploited. It's not people scuttling around hiding. I hate seeing sex scenes being done coyly, like it's not happening, or where the sheets are Sellotaped across the woman's breasts. You either do it properly and make it look like what it is, or just avoid doing it at all. I think here it's a flash of two people making love. It's not a camera lingering up my thigh."
After unambiguously graphic displays in This Life and Big Women, Nardini is gaining something of a reputation for her up-front attitude to dramatic nudity. "I read some actresses saying, `I'll never take my clothes off', and it feels like an invisible slap or a finger pointing at me - `harlot!'," she smiles. "There is a British alarm and coyness about sex. Somebody naked in a French film is not a big deal. You and I are here because people make love, and generally they take their clothes off to do so."
Nardini is going on to make a feature film, Elephant Juice, written by Amy Jenkins (the creator of This Life) and co-starring James. "I wanted a break from television. I feel that in the past few years, I've been on the verge of being overexposed," she says with a knowing chuckle. "People might start to say, `Oh no, not that Scottish girl who takes her clothes off again'."
So what role will she play when she returns to the small screen? "A Devon milkmaid who is slightly simple, who has never had sex ever and who doesn't even take her clothes off to have a bath."
`Undercover Heart' begins on Thursday at 9.30pm on BBC1
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