SINGAPORE - Melody Malone is a private detective in 1930s New York. She is also the fictional creation of River Song, a time-travelling archaeologist who is killing time in the universe's most secure prison by writing a hard-boiled mystery novel.

River, in turn, is a character on cult science-fiction show Doctor Who, played by British actress Alex Kingston - who wrote her debut novel, The Ruby's Curse, about River writing a novel about Melody.

Confused yet? "It's all a bit wibbly wobbly timey-wimey," says Kingston, 58, with a laugh, quoting Doctor Who, which she was on from 2008 to 2015.

Kingston gained fame in the 1990s through American medical drama ER, playing surgeon Elizabeth Corday in 160 episodes between 1997 and 2009.

Today, she is known for her Doctor Who role as River, a rebellious adventuress who marries the Doctor, a time-travelling alien, even though their timelines are often out of sync with each other.

Her catchphrase, whenever asked about the future she has come from, is: "Spoilers."

Though River's last appearance on the show was in 2015, she has had an active afterlife in audio dramas and now the novel, which Kingston co-wrote with Jacqueline Rayner during the Covid-19 outbreak in 2020.

"It was scary because I hadn't ever written a book before," says Kingston over Zoom. "But we were all in lockdown. I had nothing else to do."

She is not the first Doctor Who actor to write a novel featuring the character they played.

Tom Baker, who played the fourth incarnation of the Doctor, co-wrote horror novel Scratchman in 2019 with James Goss.

Kingston says she has always been fascinated by the mystery of the Egyptian queen Cleopatra's tomb.

"Nobody knows exactly what happened to her at the end and where she's buried. And Caesarion, who was her son with (Roman general) Julius Caesar, sort of just disappeared from the narrative.

"River Song is a time-travelling archaeologist and Melody Malone is a private detective who unravels mysteries. The opportunity to have the two of them working together on this in an impossible way was too delicious."

The metafictional novel blends various genres, crossing film noir with Indiana Jones-style archaeology action and shenanigans in space.

River has broken back into her prison cell in outer space so she can have peace and quiet to write her novel, in which Melody is tasked to find a cursed ruby that may hold the key to Cleopatra's tomb.

But disruption soon arrives in the form of a fellow prisoner, who knows where to find the Eye of Horus, a reality-altering device that could destroy the universe.

Forced to help him escape, River winds up travelling to 1939 New York, ancient Egypt and eventually inside her own novel, where she has to work with Melody to solve the mystery.

River - who is as handy with her PhD as she is with a pistol and can hold her own against the Doctor, with whom she flirts and fights - was popular with fans, especially female ones.

"River's moral compass is a little bit frayed at the edges. She is strong, but she also has vulnerability, deep down," says Kingston, who is married to television producer Jonathan Stamp.

Her previous husbands were English actor Ralph Fiennes and German journalist Florian Haertel, with whom she has a daughter, Salome Violetta Haertel.

"The number of stories I've heard from young people who were in hospital, and the only thing that got them through it was watching River Song episodes on a loop from their hospital beds - it's really humbling, particularly when I didn't have the faintest idea that that was what I was doing when I acted in the episodes."

Asked if she might ever return to the show, as previous beloved characters have done in recent seasons, she merely laughs. "Spoilers."

• The Ruby's Curse ($40.67) is available here