This series is a sequal to Life on Mars which story naturally ended at the end of season 2.

Ashes to Ashes is set in the 1980's rather than the 1970's of Life on Mars.

The idea of Ashes to Ashes is almost the same as Life on Mars about a modern day police officer (this time a woman rather than a man) getting hurt and going into a coma which causes them to experience life in the past.

The main difference about Ashes to Ashes over Life on Mars is that it has more focus on DCI Gene Hunt (a character from the past) and slightly less on the police officer who is hurt and in a coma.

I have seen every episode of series 1 and although it isn't quite as good as life on Mars, DCI Gene Hunt does really carry it well and it is very interesting how they fit a modern day police woman into the 1980's world.

Again here is the Wikipedi bit for it.

Ashes to Ashes features DCI Gene Hunt (played by Philip Glenister) from Life on Mars. He is joined by an experienced female detective, and trained psychologist from the 21st century, DI Alex Drake (Keeley Hawes), who wakes up in 1981 after being shot in 2008. She concludes that she is either a second away from life, or a second away from death, because of the detailed notes she has studied from Sam Tyler, when he found himself back in time in 1973 after a car accident.^[5] Also reappearing are Chris Skelton (Marshall Lancaster) and Ray Carling (Dean Andrews). All have transferred to the Metropolitan Police in London, and Chris and Ray remain loyal to Gene. Augmenting the team is WPC Sharon 'Shaz' Granger, (Montserrat Lombard).