It is based around two characters from EastEnders, Kat (Jessie Wallace) and Alfie Moon (Shane Richie), who travel to the village of Redwater in Ireland, where the Kelly and Dolan families have lived for generations.
These are the family trees for the characters in Kat & Alfie: Redwater, as understood at the start of the series.
[2] McElhinney described Lance as a "patriarch" but "quite weak" while his wife, Agnes, "rules the roost", and "he's quite happy to be a character in the town with the kind of respect that comes from having been around for quite a while being the 'grand old man' of the village.
In the present day, Lance discovers that Kathleen has come to Redwater to find her son who was adopted 33 years ago.
[5] Doyle Kennedy said that they were in love once and had a child very young, and felt that the relationship then was a "youthful, possibly impetuous courtship and decision to marry that maybe hasn't worked out quite the way they thought it would.
A nun, Sister Ruth (June Whitfield), explains that Kat's family, fearing that the child would turn out like his paedophile father, Kat's uncle, Harry Slater (Michael Elphick), handed him to the nuns in the convent where he was born.
21 years later, Kathleen and Alfie arrive in Redwater, where Kat is attempting to find her son.
Dermott is angry at being delivered orange juice by the local dairy farm when he has not requested it.
Agnes insists that nothing is said, but Lance tells Dermott that Iris and Peter are not his parents and that Iris lost her baby while staying in London, so brought back Kathleen's son as her own, and Peter does not know the truth.
The next day, after finding more orange juice has been delivered, Dermott meets Lance while Lance is swimming in the sea and makes him eat a sacramental wafer and then drowns him in the sea while flashing back to the tragedy, where he lets go of Iris's hand, allowing her to die.
It was decided that Kat should be shown as a survivor and move on to the next stage of her life, and she was taken back to "Kathleen", the child she was before she was abused.
[13] In some of Kat's final EastEnders episodes, she discovers that she gave birth to a son, Luke, after she passed out during the birth of her daughter Zoe Slater (Michelle Ryan); Luke was adopted by a family in Ireland without Kat knowing he existed.
Treadwell-Collins confirmed that "the discovery that Kat has a secret son will [...] ultimately lead to the Moons realising that the answers to so many questions lie across the sea in Ireland.
"[14] Director Jesper W. Nielsen said the series is "About how one woman's search for a lost child opens Pandora's box; revealing the terrible lies and secrets in the little village of Redwater".
[18] Wallace described Kat as a "colourful person" with "a lot of layers to her" and said that after playing her for 16 years "means there's plenty underneath to bring to the surface.
[19] Wallace performed some of her own stunts in the series, including jumping from a boat into the sea and an underwater sequence.
Richie later said, "For the first time in 15 years, [Alfie] questions himself as a father, a husband and as a life, it feels like it's coming to an end for him.
Doyle Kennedy described Roisín as "quite complicated", different on the outside to how she is inside, "very practical, very stoic and keeps a lid on most of her dreams and passions as there's not much room for them in her life."
[5] Doyle Kennedy said that Roisín's relationship with Eileen is "interesting" and in reality, she and Angeline Ball, who plays her, have been friends for many years.
[5] On Kathleen and Alfie's arrival, Doyle Kennedy said that Roisín is "blasé" about them as "People often come to Redwater on holiday" but sees them as "unusual" and is not suspicious of them.
[24] Ateh, who lives in Los Angeles, was told about the part by a friend who saw an advert looking for a mixed race actress.
She called the character "a fighter and a survivor" and explained that she left Redwater 20 years ago, which was "a difficult decision", and moved to the United States with her young son.
[22] Eileen's return to Redwater makes her "less polished, less 'New York'", and her American accent drops away, as does her "hardened exterior".
[22] Eileen left Redwater because of her grief after she lost a child to drowning, and because for independence from her "a very strong mother and sister".
[22] Doyle Kennedy said that Roisín's relationship with Eileen is "interesting" and in reality, she and Ball have been friends for many years.