This three-part psychological thriller set in Glasgow follows the paranoia of a woman who sees her identity being dismantled and can't work out whether the cause is another woman or her own baby.

Ellen gets unexpectedly pregnant just after landing a big contract for her architects' practice. She hopes to be away for the shortest time and hires someone to cover the gap. Paula is the same age as Ellen but already has a ten year old daughter. She made the opposite life choice, took a big chunk of her career out to dedicate herself full time to her child.

As Paula shadows Ellen for the last months of her pregnancy, she at first seems the perfect hire - enthusiastic, personable, super-competent. But Ellen soon begins to worry Paula has another agenda. Paula refuses to believe Ellen when she says she plans to return in a couple of months. She's confident Ellen will change her mind when she falls in love with her baby. Ellen thinks that maybe Paula doesn't want her back. Ever.

Ellen becomes increasingly disturbed by Paula's subtle manipulations at the office. She sees Paula ingratiate herself with bosses and client, move in on her friends. Ellen begins to see her less as a support and more of a rival. But no-one else sees the Paula Ellen sees. They see a lovely bright decent woman doing her best to cover for Ellen. They all think Ellen is being paranoid, even her own husband doubts her state of mind. She is after all pregnant.

Only Ellen can see that Paula seems more obsessed about the pregnancy than she is. And the more she voices her fears, the more she alienates everyone around her. When a terrible accident strikes the firm, Ellen cannot accept the police verdict and pursues the person she believes responsible.

• Starring... Morven Christie and Vicky McClure
• Written and directed by... Joe Ahearne

• Produced by... Nicole Cauverien for Left Bank Pictures

Introduction by writer and director Joe Ahearne

I like stories about paranoia where our main character is the only one who can see what's going on while everyone around them thinks they're going crazy. This type of story gives you the opportunity to photograph a character thinking not just talking and makes for much more visual and compelling drama.

Someone going on maternity leave and having to deal with a temporary replacement seemed like a great way to explore this emotion. She watches what's going on around her and sees evidence everywhere she looks that she's being undermined. Everyone can identify with the anxiety of seeing someone do their job as good or better than you can. The pang when a colleague invites another colleague for lunch and doesn't tell you. Most of us have the secret fear that we're not good enough and feel envy when we see others navigating professional and social situations more suavely than we think we can manage.

The Replacement turns that up to 11. The fact it's a pregnant woman going through this intensifies the dilemma. Women get attacked from every side when they make decisions about how to manage the change in their life. Whatever decision they make is challenged and dissected. It's the perfect landscape for paranoia and suspense.