Here's a summary of the series premiere of Life on Mars:
In 2008, NYPD detectives Sam Tyler and Maya Daniels are close to catching murderer Colin Raimes. After Raimes is released - despite a lame alibi - Daniels decides to track him down without her partner - and then she goes missing. Tyler desperately looks for her, but is struck by a car and mysteriously wakes up 35 years earlier... in 1973.
Once there, the new precinct treats Sam like a regular cop, not buying any story about time travel.