Claire Foy has been cast in 'The Pisces'.

The 36-year-old actress will star in Gillian Robespierre as PhD student Lucy – who is plagued with writer's block and romantic obsession.

'The Pisces' is based on Melissa Broder's critically acclaimed novel of the same name and centres on Lucy's erotic infatuation with a merman. The flick is more than just a romance between lady and fish-man, as it explores the boundaries between pleasure and possession, fantasy and reality and the way women choose the men in their lives.

Melissa and Gillian have written the script while Anne Carey and Amy Nauiokas are producing under their Archer Gray banner.

Claire is also set to star in an adaptation of the novel 'Migrations' alongside Benedict Cumberbatch.

'The Crown' actress is also executive producing the flick which is based on Charlotte McConaghy's book.

The story centres on Franny Stone, whose dark past of love, loss, crime and motherhood unfolds on an odyssey through stormy seas with a team of misfit fishermen.

Claire said: "Charlotte has created an utterly unique and beautiful story with a compelling and fascinating woman at its heart. I couldn't be more honoured or excited to help bring this truly remarkable novel to the screen."

Producer Leah Clarke added: "We have been looking for an opportunity to collaborate again with Claire on something special that explores big themes and offers Claire an opportunity to portray a character the likes of which we have not seen before.

"We are delighted to have found it in Charlotte's undeniably cinematic 'Migrations' and the character of Franny Stone. Together we all hope to bring you something very special."