Watch: Vicky McClure and Morven Christie in trailer for 'The Replacement'

The BBC have released the trailer for upcoming drama The Replacement starring Vicky McClure and Morven Christie.

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.

The Replacement starts soon on BBC ONE.