Sam Tyler is a detective that gets hit by a car... and instantly transported back to 1973.
He wakes up during the age of the Vietnam War, Watergate, women's liberation and the civil and gay rights movements. He has no idea how he got there and no one really believes his tale.
As a result, Tyler is thrown right back into the investigative scene, without the help of modern day devices that he had grown so accustomed to. He ends up on the case of a serial killer, who may actually play a role in his girlfriend's kidnapping from the present day.
Sam: (sarcastically) Right, that must be why I'm here.