On the season premiere of CBS's Blue Bloods, titled Unwritten Rules, an armed robbery ends with the death of a police officer and Frank Reagan (Tom Selleck) is determined that the NYPD bring the suspect to justice.

A promo clip for the premiere showed a female officer getting Jamie Reagan (Will Estes) as her first partner ever but dying shortly after when out on assignment. Tensions rose in the Reagan household when Erin Reagan-Boyle (Bridget Moynahan) dropped the charges and released the suspect due to lack of evidence. However, she admitted that she does want to get the suspect and told Danny Reagan (Donnie Wahlberg), "I want him as badly as you do."

The promo video for the season premiere, shown below, revealed that the series returns with "a new beginning, a heroic ending and everything you love about New York's finest."

Frank looked distraught when he arrived at the hospital to find that the police officer died only a few minutes ago. He then made a phone call explaining how he is making it his mission to find the suspect. He told the person on the other end of the call "[we] got an officer in the morgue and a cop killer on the street."

He is later heard saying, "When one of our own goes down, we step it up a notch, especially this family."

Danny is then shown chasing a man wearing a backpack through Grand Central Station and then tackling him down to the ground.

Season 4 of Blue Bloods premieres Friday, Sept. 27 at 10 p.m. ET on CBS. Following a recurring guest starring role at the end of season 3, Marisa Ramirez was promoted to series regular in June. She will continue to play Detective Maria Baez, Danny's former and now full-time partner. She is a permanent replacement for Jennifer Esposito.

Tags: Blue Bloods CBS