But one day, the body of an American student turns up. Her name is Mahalia Geary and her murder has huge implications for both cities and for the man who investigates the killing: Inspector Tyador Borlu.
Here are the characters you'll meet, and where you've seen them before:
Tyador Borlú – David Morrissey
Who does David Morrissey play? Inspector Tyador Borlú belongs to the Extreme Crime Squad in the fictional European city-state of Beszel. He is assigned to investigate the murder of Mahalia Geary (Andrea Deck), a woman found dead and badly beaten in a Beszel street. But there is more to this murder, and it involves his city's "twin city", Ul Qoma, which shares the same geographical space but with strict boundaries that cannot be transgressed.
What else has David Morrissey been in? Having broken into the industry at the age of 18 when he starred in One Summer (1983), David Morrissey has starred in movies including Our Mutual Friend, Hilary and Jackie, Some Voice, and Captain Corelli's Mandolin. On TV he has played Gordon Brown in The Deal, Aulus in Britannia, and Sam in The Missing. American fans will probably know him as the Governor in The Walking Dead.
Lizbyet Corwi – Mandeep Dhillon
Who does Mandeep Dhillon play? Lizbyet Corwi is the police officer appointed to the Mahalia Geary case alongside Inspector Borlú.
What else has Mandeep Dhillon been in? You may recognise Mandeep Dhillon from the TV series Damned, or The Good Karma Hospital. Her other roles have included Shaz in Hospital People, and Shireen in Doctor Who.
Katrynia – Lara Pulver
Who does Lara Pulver play? If you don't remember Katrynia from the book, you're not going mad. The TV adaptation has added a personal storyline for our protagonist Borlú, giving him a wife who disappeared – apparently "breaching" from Beszel into Ul Qoma and breaking the rules. In the first episode we see her in flashbacks.
What else has Lara Pulver been in? Lara Pulver played fan favourite Irene Adler in Sherlock. She has also starred in True Blood and Da Vinci's Demons, and recently appeared in the Electric Dreams episode Real Life.
Quissima Dhatt – Maria Schrader
Who does Maria Schrader play? Originally portrayed as a man in the novel, senior detective Dhatt is Borlú's counterpart in Ul Qoma.
What else has Maria Schrader been in? German actress Maria Schrader is best known in the English-speaking world for her role as Lenora Rauch in Deutschland 83 and Deutschland 86.
Commissar Gadlem – Ron Cook
Who does Ron Cook play? Commissar Gadlem is Borlú's boss on the Extreme Crime Squad in Beszel.
What else has Ron Cook been in? You may recognise him from Mr Selfridge, where he played Mr Crabb. Also known for his work in the theatre, Ron Cook has starred on TV in Little Dorrit, The Diary of Anne Frank, Funland, and The Singing Detective.
Yorju Syedr – Danny Webb
Who does Danny Webb play? Politician Yorju Syedr is a Beszel nationalist and a dangerous man.
What else has Danny Webb been in? Last year Danny Webb starred as DS Rory Maxwell in ITV's psychological thriller Liar. He's also been in SS-GB (as Sydney Garin), The Halcyon (as Mortimer) and Humans (as Hobb), and back in 2006 he guest starred as Mr Jefferson in Doctor Who.
Naustin – Dana Haqjoo
Who does Dana Haqjoo play? Inspector Naustin helps to examine Mahalia's body and questions witnesses.
What else has Dana Haqjoo been in? You may recognise Dana Haqjoo from his roles in The Omid Djalili Show, or as a police officer in Emmerdale.
Yolanda Rodriguez – Morfydd Clark
Who does Morfydd Clark play? Mahalia's best friend. She believes that she, too, is in danger.
What else has Morfydd Clark been in? Welsh actress Marfydd Clark has appeared in the films Interlude in Prague, Love and Friendship, and Pride and Prejudice and Zombies. On TV she will play Debbie in upcoming Benedict Cumberbatch drama Melrose, and she's also been on stage in productions including Romeo & Juliet and The Cherry Orchard.
Rebecca – Marama Corlett
Who does Marama Corlett play? Rebecca's role is still unknown.
What else has Marama Corlett been in? You may recognise Marama Corlett from Sick Note, where she played Linda, or Blood Drive, where she played Aki. In 2014 she made a brief appearance in Guardians of the Galaxy as a pit boss.