Joined onscreen by Whitechapel's Rupert Penry-Jones, the twosome are set to play Jodie and Jason respectively.
The characters are siblings who have always depended on one another due to their fractious relationship with their mother Lynn, played by Deborah Findlay.
Having painstakingly got her life back on track since the death of her son eight years ago, Jodie finds everything she's built back up again threatened when she becomes fixated on a young boy named Daniel (Cody Molko).
Elsewhere, practising lawyer Jason is on the cusp of getting an important promotion.
Rupert-Jones plays Daniel's father Mark in the Dublin-set four-part drama, which is expected to air later in 2020.
"We are delighted to be working with the hugely talented Jill Halfpenny, Jonas Armstrong and Rupert Penry Jones on The Drowning, which is being lead by the brilliant director, Carolina Giammetta," executive producers Noel Clarke and Jason Maza said in a statement.
"Working with Carolina and our wonderful writer, Tim Dynevor, the creative process has been a joy.
"We could not be more excited to collaborate with Channel 5 to bring this story to the screen."
Sebastian Cardwell, Channel 5 Deputy Director of Programmes, added: "We are thrilled to be working with the team at Unstoppable on this exciting addition to Channel 5's ever growing drama slate.
"We hope that audiences will be gripped across the series."
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