Every so often, there comes a shocking or unexpected moment in a television series that leaves the show forever altered. These show-changing moments are not simply unique plot twists or ambitious story arcs; they are episodes or scenes that ultimately prove to be watershed moments for the series in which they occur. The impact of a show-changing moment on a series can be positive or negative - and sometimes a verdict can't be reached until long after the show has ended.

We have comprised a list of the top five such moments. The way in which each of these instances affected the five series may vary, but there is no denying the overall transformations that each one ultimately brought about.

Alias (Takedown of SD-6)

The premise of "Alias" for the first season and a half revolved around Sydney Bristow's (Jennifer Garner) life as a double agent. Prior to the start of the series, she had been working for SD-6, which she thought was a covert branch of the CIA. In the pilot episode, Sydney was approached by the real CIA and informed that SD-6 was actually a part of the very terrorist organization she thought she was fighting. Needless to say, she was left pretty red-faced (not to mention a little steamed). The series proceeded to focus on Sydney's dangerous role as a secret operative trying to sabotage SD-6 from within.

That all changes with the episode "Phase One." In that episode (which aired midway through season 2), the U.S. government raids and completely destroys SD-6 headquarters. In that moment, the entire premise of "Alias" is shattered, and the story begins anew. While some might argue that the change was for the better and others that it was for the worse, there is no denying the fact that "Phase One" completely altered the premise and structure of the series.

Friends (Monica & Chandler in London)

The early seasons of "Friends" too often revolved around the poorly matched pairing of Ross (David Schwimmer) and Rachel (Jennifer Aniston), two characters who never really felt right together as an on-screen couple. And then there was London. In the season four finale, the central characters travel to the English capital in order to celebrate the blessed (though short-lived) union of Ross and Emily (Helen Baxendale).

But it is not Ross' marriage to Emily (or the fact that he accidentally said Rachel's name at the altar) that is the truly memorable moment in this episode. Instead, it is the shocking reveal of Monica (Courteney Cox) and Chandler (Matthew Perry) in bed together that truly steals the show. Their relationship would continue to develop after a return to the states and became a driving force of the series' narrative throughout the later seasons. The Monica and Chandler relationship effectively shuffled the living arrangements and relationships of all six main characters and proved to have a much stronger affect on the series than the typical romantic pairing often does.

The coupling might have seemed a little odd at first, but it quickly became apparent that Monica and Chandler had something going for them that Ross and Rachel never did. They had chemistry.

Twin Peaks (Laura Palmer's killer is revealed)

"Twin Peaks" was a complex, multilayered and decidedly odd character-driven drama that featured an enormous cast of characters. While each character had his or her own storyline chock full of secrets and intrigue, everything was seemingly tied together by the Laura Palmer murder mystery. It was certainly a compelling story arc, and it even briefly gripped the nation in a way reminiscent of the "Who shot J.R.?" craze several years earlier.

A murder mystery can only sustain itself for so long (in this case 17 episodes) before it invariably leads to a revelation. The reveal of Laura's killer was a fantastic and riveting piece of television, but unfortunately, everything went downhill from there. With the central storyline in the series resolved, the narrative quickly lost focus and the episodes became increasingly bizarre (even by "Twin Peaks" standards). Audiences departed in droves, and the series was laid to rest shortly thereafter.

M*A*S*H (The 4077th learns of Col. Blake's death)

The first few seasons of "M*A*S*H," while occasionally tackling the horrors of war, were mostly lighthearted and comedic overall. All of that changed in the closing moments of the third season finale. Much of the episode had focused on Col. Henry Blake (McLean Stevenson) preparing to depart Korea and return to civilian life, his obligation to the U.S. military having been fulfilled. After sharing some tearful goodbyes, he boards an Army helicopter and rides off into the metaphorical sunset. Things at camp return to normal, and all is well and good until Radar (Gary Burghoff) bursts into the OR and shares some devastating news with the 4077th. "[Blake's] plane was shot down over the pacific, there were no survivors."

The scene is one of the most iconic in television history and was truly groundbreaking at the time. In addition, it ultimately signified the beginning of a major change in the tone and scope of the series. In the seasons following that tragic moment, "M*A*S*H" drifted away from the early seasons' emphasis on humor and began instead to focus on dramatic topics regarding the tragedies of the war. The men and women of the 4077th were never quite the same after Blake's death; and neither was the series.

Lost (Flashforward reveal)

In one of the most surprising and unexpected television moments of all time, it is revealed that Jack's (Matthew Fox) "flashback" scenes in the third season finale were in fact a "flashforward." It was a breathtaking revelation for most fans of "Lost," who after three seasons had become accustomed to the flashback method of storytelling that the series employed. The introduction of the flashforward added a whole new perspective.

In addition, the flashforward revealed that yes, at least some of the Oceanic 815 survivors do end up leaving the island. And something happened to them once they did, something that forced Jack, ever the skeptic and "man of science," to have a major change of heart. If its intended effect was to get people to start questioning what they thought they knew, then it succeeded. Bravissimo.

The flashforward completely changed the rules of the series, and also reaffirmed the fact that showrunners Damon Lindelof and Carlton Cuse have a definite long-term story structure in place, something that seemed somewhat dubious at times throughout the spotty third season. There is still a long way to go, but when the series is complete, this moment could go down as the biggest show-changer in television history.

Dishonorable mention #1: Roseanne

The completely unnecessary ninth season ruined the otherwise strong legacy of the sitcom about a working-class matriarch and her brood. The Conner family's lottery win early in the season completely altered the premise of the series and was just the first of many ridiculous events to occur throughout this disaster of a final year.

Dishonorable mention #2: Coach

"Coach" did more than just share an hour with "Roseanne" on ABC's schedule; it also shared an unfortunate, late series reboot. After seven seasons as a college football coach in Minnesota, Hayden (Craig T. Nelson) received an offer for a pro job in Florida. He moved down to the Sunshine State (with all of the regulars in tow), for two painful seasons under the sun. The series was on its last legs prior to the relocation, and this served as the nail in the proverbial coffin.

Were your favorite show-changing moments on the list? What are they? Let us know!

Story by Derek Krebs
Starpulse contributing writer