Fox's latest ad for hit animated sitcom Family Guy is unusually somber. "There is nothing on this earth more to be prized than true friendship," reads a quote by theologian Thomas Aquinas over a syrupy string orchestra.

(Beware: Spoilers lie ahead.)

This isn't exactly business as usual for a series best known for its edgy, adult-oriented slapstick humor. So who merited this touching tribute? Brian Griffin, the anthropomorphic dog that has served as one of Family Guy's main characters since the animated comedy premiered on Fox in 1999. In last night's "Life of Brian," the Griffin family's dog was hit by a car and killed — and while the show has played fast and loose with death before, the twist came just after Stewie Griffin dismantled his time machine, rendering Brian's death irreversible. "You've given me a wonderful life. I love you all," said the bruised and bloodied dog as he lay dying on an operating table.

The events of "Life of Brian" were actually teased all the way back in July, when showrunner Steve Callaghan revealed he planned to "kill off a member of the Griffin family and replace them with a different character." But few predicted that the character would be a fan favorite like Brian, who has remained one of the few relatively bright spots in Family Guy's increasingly dismal run. By the end of the episode, the character — voiced by series creator Seth MacFarlane — had been replaced by a new dog named Vinny, voiced by Sopranos alum Tony Sirico.

Deaths of major characters are unusual on any TV series — but particularly in an animated series, where the departure or death of a voice actor is the likeliest reason to retire a character. On Family Guy's sister show The Simpsons, Maude Flanders was killed off after voice actress Maggie Roswell quit the show in 2000; earlier this year, The Simpsons announced the retirement of Bart's teacher Edna Krabappel after the death of voice actress Marcia Wallace.

Unlike either of those situations, Seth MacFarlane — who also voices main characters like Peter and Stewie Griffin — seems likely to remain with the show for the foreseeable future. So what prompted the shakeup at Family Guy? In an interview with E! Online, Callaghan said the original pitch for the episode "caught fire" in the writer's room. "As soon as this idea came up, we started talking about what the next couple episodes could be and we got very excited about the way this change will affect the family dynamics and the characters." Callaghan adds that next week will be a "regular, run-of-the-mill episode," and that "fans are smart enough and loyal enough" to trust that Family Guy's writer's "always make choices that always work to the greatest benefit of the series."

Still, fans are clearly skeptical that Brian's death will be permanent given Family Guy's freewheeling approach to continuity. As Eric Thurm notes at The AV Club, Meg Griffin died at the end of an episode "just a couple of weeks ago," and the current top comment on the YouTube tribute speculates that the show's creative team is "just trolling."