CBS this morning confirmed that CSI: Crime Scene Investigation will end its 15-season run with a two-hour finale, featuring original stars William Petersen and Marg Helgenberger. The two-hour movie will air Sept. 27, the first Sunday of the season. Additionally, current CSI leading man Ted Danson will be transitioning to new spinoff CSI: Cyber starring opposite Patricia Arquette as a new regular. He will reprise his character from the mothership series, D.B. Russell.

After concluding that CSI and CSI: Cyber could not both exist on the schedule next season, CBS’ Nina Tassler said the network considered various options for a sendoff to the mothership series, including a six-episode order, but she said that didn’t have the event feel a two-hour finale would have. (I hear the idea to go out with a two-hour finale came from CBS Corp. CEO Les Moonves).

It is unclear yet whether there will be casting changes on CSI: Cyber in light of Danson’s arrival, Tassler said. (UPDATE: I have learned that CSI: Cyber regular Peter MacNicol will not return for Season 2. Here is my story.) The series, which had a so-so freshman run, had been expected to undergo creative tweaks heading into Season 2. On CSI: Cyber, Danson and Arquette’s characters, D.B. Russell and Special Agent Avery Ryan, already have a history together as CSI: Cyber was introduced in an episode of CSI last season.

CSI has been one of CBS’ most important series of the past two decades. It is bowing out after 15 seasons and three spinoffs, having established a Thursday stronghold for the network at a time when NBC was dominating the night and having delivered a financial windfall for the studio by launching a $1 billion franchise.