Alternate Reality Episode

An Alternate Reality Episode is slightly different from an episode that incorporates an Alternate Reality.

In an Alternate Reality Episode, the usual main characters are absent. The main cast is still here, but they're playing "themselves" in a different role. Maybe Alice is now a Dot Com boom millionaire instead of a lowly research assistant, or the other way around. The point is that it's our heroes being different people, not the same people visiting a different place.

Typically, the "normal" situation may get a look in for a couple of scenes at the start or end of the episode, but there should not be any clear link between the characters, Alice the millionaire should not wake up as Alice the Research Assistant, or have Bob her millionare buddy visiting her, asking why everything is different.

Compare with Parody Episode (this trope could be considered the serious side of the coin) and Elseworld.

Examples of Alternate Reality Episode include:

Live Action TV

Friends: "The One That Could Have Been" is a two-parter where the teaser has each of the Friends imagining something that could have happened differently in their lives and the rest of the story drops into an Alternate Reality where these things happened. Monica never lost her teenage weight and is still fat as an adult, Joey is still on Days of Our Lives and has become famous and wealthy, Rachel married Barry and is quietly miserable (and considering having an affair with Joey the celebrity), Pheobe is a stockbroker, Chandler is a (failing) writer and Carol has not come out of the closet and is still married to Ross (Though she is very interested in having a threesome with Ross and another woman).
The Star Trek: Enterprise episode "In a Mirror Darkly" is an Alternate Reality Episode down to having a different Title Sequence, whereas "Mirror Mirror" and the DS:9 Mirror Universe Episodes are not, instead featuring one or more of the regular cast actually crossing over and interacting with the Mirror Universe.
Played terrifyingly in Buffy the Vampire Slayer: bitter over her failed relationship with Xander, Cordelia makes an idle wish that Buffy had never come to Sunnydale, believing that her popular status would still be intact if she had never gotten involved in the Scoobies' affairs; unfortunately, she speaks in front of a disguised vengeance demon who grants her wish to the letter. Cordelia is tossed into a crapsack alternate universe where the Sunnydale population is a tenth of what it was, due to unchecked vampire attacks, due in turn to the Master having ascended a year and a half before without Buffy there to stop him. Cordelia is killed by evil vampire versions of Willow and Xander, Giles and Oz are trapped in thankless work as desperately outnumbered vigilantes attempting to do what they can to restore some semblance of safety to the community, Angel is kept in a cage with the Master's minions allowed to torture him for fun, and Buffy eventually makes an appearance as a hardened, pitiless rogue Slayer who has gone off the deep end without her friends' humanizing influence. The episode ends with a vicious Final Battle in which the entire main cast kills each other; Giles' last-minute actions save the day and propel everyone back into their proper reality, but it's still incredibly frightening.
- Played with in another episode where Willow casts a spell intended to remove Tara's memory of breaking up with her but instead blanks out everyone's memories including her own. They're in th same universe but because they're all starting from a blank slate they reach some very different conclusions about themselves. Giles and Anya think they're married, Spike thinks he's a vampire with a soul, Buffy finds being a Slayer cool etc.
- The spin off Angel had "Birthday" where Cordelia got a chance to make her life what it would have (or should have according to the one offering her the choice) been like if she had met a big-time talent agent instead of Angel in the pilot. She becomes famous but Angel gets the visions (because Doyle still died) and it drives him mad. Wes and Gunn are with him, but with only three arms between the two of them.
The Stargate Atlantis Season 5 episode "Vegas" takes place in an Alternate Reality where John Sheppard is a Las Vegas detective and never joined the Atlantis expedition. There is a brief crossover (a message sent from that universe enters the main one), but no characters crossover. It is sometimes referred to as CSI: Atlantis, which was actually its working title during production.
Half of each episode of Lost Season 6 is devoted to a alternate reality where the plane never crashed, which apparently resulted from the cast's attempt to change history in season 5, while the other parts of the episode continue as normal. The two timelines seemed totally independent for a while, but now certain people in the "flash-sideways" timeline, particularly those with love interests, have begun having visions of the island timeline...
- Ultimately, the alternate universe proves to be a cross between a Dying Dream and Mundane Afterlife where nothing up to that point actually mattered.
Bones did one in which Booth and Brennan were married, and owned a nightclub (called "The Lab") where many other series regulars worked. A murder takes place there, and we get to see Brennan squicked out by death. This is actually a dream of Booth's, while he is in a coma. It's actually his perfect world and based on Brennan talking out loud to him in his coma while she works on her next book.
In the two-part 3rd Rock from the Sun episode "Dick'll Take Manhattan", the aliens go through a time-space portal to a parallel universe where they lead upscale lives in New York City. Dick is a lawyer, Sally is a columnist in the vein of Sex and the City, Tommy is a cast member on Saturday Night Live (with then cast members Tracy Morgan, Ana Gasteyer, and Darrell Hammond appearing as themselves), and Harry is the president of NBC. Other characters from the show turn up as well, all leading different lives of some description. Inevitably, the Solomons return home after finding their new lives to be shallow.
The Doctor Who episode "Turn Left" is created when time-line interference makes a universe where the companion Donna Noble never met the Doctor, and as a result, he died. A space-ship attack at Christmas meant that Donna lost her job, although she did win a prize out of town the next holiday season. Good thing, too, because without the Doctor there to help London was nuked and everyone in the city died. Then It Got Worse. The Doctor and Donna weren't there to stop the adipose, and tens of millions of Americans were killed. The Sontarans nearly succeeded in rendering the planet inhospitable, saved only by the heroic sacrifices of the Torchwood team. And then all of reality began to destroy itself....
Xena: Warrior Princess saved the Fates in "Remember Nothing" and was given a chance to live her life if she had never become a warlord and "When Fates Collide" when Caesar forces the Fates to change his fate making him and Xena rulers of Rome.