However, sometimes, a character is only scared of something for one episode. Maybe they've always been scared of it or have been scared of it for a while, but it's only cropped up in that episode because it simply wasn't around in previous episodes. Maybe they developed a fear over the course of that episode, but whatever the case, they are never shown or said to be afraid of it before and they are never shown or said to be afraid of it since. If someone mentions in later episodes as a Continuity Nod that the character used to be afraid of it, it still counts, but the character needs to only actually be afraid of it for one episode for it to qualify.
The episode will usually focus on the fear, either as a main plot or a subplot, and usually the focus is on either facing it or making it go away, whether because the character has to do what they're afraid to do (for example, Alice Speaks Fluent Animal but is afraid of lions and she and the others have to face up to a lion) or they recently developed the fear and it's related to something they do all the time (for example, Alice becomes afraid of cats and has to get rid of the fear so she can play with Bob's cat, which they like doing). At the end of the episode, the fear will either already be gone, or the character will state or imply that they're almost not afraid of it anymore (and will effectively not be afraid of it anymore come the next episode, or else it won't be this trope).
The One-Episode Fear can also count as a Compressed Vice if the character is stated to have always had it but previous episodes prove otherwise, or an Informed Flaw if they're mentioned in passing to be scared of something in one episode but act like they're not. Compare Idiot Ball (and its subtropes Villain Ball and Hero Ball) and Smart Ball for other personality traits that disappear as quickly as they appear. May result in Conflict Ball if the fear causes a conflict and may be a form of Backstory of the Day if it's supposed to be that they've always had the fear. Multiple characters revealing a new fear may lead to a "What Do They Fear?" Episode.
- In Diary of a Wimpy Kid: The Ugly Truth, Greg the main character says that he is nervous around eggs because of an incident where he dumped devilled eggs into his neighbours' pot plant, leading to them rotting, the neighbours not being able to identify the smell, and moving out. He's never been shown or stated to be afraid of eggs before or since.
- Mr. Men: In her title book, Little Miss Somersault discovers she is afraid of heights, which doesn't show up in any of the subsequent books, or the cartoon.
- Charmed: Barbaras makes Prue afraid of water in one episode, which she gets rid of of her own accord.
- In an episode of Happy Days, Arthur Fonzarelli, "The Fonz" himself, is revealed to have a phobia... of liver. This fear is exposed by none other than his best friend Richie Cunningham, in the course of an investigation for a news story. Richie's journalistic integrity won't allow him to squash the story, but his friendship with Fonzie compels him to help his buddy conquer his fear, which he does, making it a non-issue.
- M*A*S*H: Hawkeye having Claustrophobia is a major plot point in the Season 7 episode "C*A*V*E", but is never mentioned at any other point in the series.
- Sesame Street:
- In the episode "Afraid of the Bark", Zoe becomes scared of dogs after Barkley gives her a fright and it goes away by the end of the episode.
- In another episode, Elmo becomes scared of fire due to a fire in the convenience store.
- Star Trek: The Next Generation: The episode "Realm of Fear" focuses on a secondary character Reg Barclay's fear of transporters, that goes away by the end of the episode. Reg is quite nervous in general, but this is the only time he was afraid of transporters.
- Star Trek: Voyager: In "One", Tom Paris has to be in stasis but keeps getting out because he has Claustrophobia (because closed spaces remind him of coffins, apparently) and he apparently has always had it, but it's never been relevant before or since.
- 21 Jump Street: When Penhall and Hanson go undercover in a juvenile prison, Penhall reveals to both the viewer and his best friend that he is claustrophobic and gets shipped out. In a later episode, Penhall is in a very small prison and not claustrophobic at all.
- In "Shelter From the Storm", the Brain develops a fear of wind after the titular storm. Leans slightly more towards Played for Drama than other examples, in that he required a psychiatrist, but it's gone by the end of the episode.
- In "April Ninth", Arthur becomes scared of being separated from his father after the father was in a fire. The fear goes away after a pep talk from his dad and a story about a similar experience he had with Grandma Thora being in a car accident.
- In "D.W. All Wet", D.W. becomes so scared of octopuses that she doesn't want to swim. She stops being afraid of them by the end of the episode, and it hadn't shown up previously because that was the first time she'd seen an octopus. She does use a lake of octopi as an obstacle when she writes a story in a later episode as a call-back, but isn't scared of them herself anymore.
- Big City Greens:
- The episode "Hurt Bike" has Cricket develop a fear of doing anything reckless after almost getting killed in an accident on the dirt bike Nancy gave him.
- "Urban Legend" has the town fear Gramma Alice because they think she's a wicked swamp witch. They obviously had no fear of her before, or since that episode.
- There are two in Jelly Jamm. The characters get over their fears in the episodes that introduce them, so this might be justified.
- Goomo's fear of burst-into-song flowers is only brought up in "Flower Fear".
- "Royal Roommate" is the sole mention of the King being afraid of the dark.
- Little Princess: The Queen is scared of heights in "I Want to Be a Queen", which seems to surprise her because when she initially climbs the tree, she doesn't know why she's scared due to thinking she isn't scared of heights, but then she concludes that she is. When the Princess tells her the fear will go away, it does.
- "Down the Drain" focuses on Tommy (who has recently developed the fear) and Chuckie (who used to have the fear, then it went away and came back) being afraid of being sucked down the drain (they're little kids).
- In "The Slide", Chuckie becomes afraid of slides for the episode after being pushed down a too-big slide. His fear goes away after Susie tells him to face it by saying he's a big, brave dog.
- In "The First Cut", Tommy cuts his finger on a bush while trying to return a baby bird to its nest and becomes afraid of getting injured, scared that he'll cut himself again and all the "red stuff" will come out of his body, to the point that the normally fearless tot freezes up when he and the twins think Spike is in trouble. Chuckie (who had been away most of the episode) helps Tommy get over this fear by showing him that his cut will heal over time.
- The Simpsons: The episode "Fear of Flying" deals with Marge's fear of, well, flying. While it is justified that it doesn't show up in later episodes, because her therapist helps her come to terms with the phobia, which could well have made it go away, it's odd that it didn't show up before, because she travels by plane in previous episodes such as "My Lisa Goes to Washington", and the incident that made her afraid of flying happened in her childhood.
- SpongeBob SquarePants:
- In "Fear of a Krabby Patty", SpongeBob somehow develops a fear of Krabby Patties for the episode after working 24/7 at the Krusty Krab for 43 days.
- In "One Coarse Meal", Plankton is extremely afraid of Pearl because she's a whale (they eat plankton like him). He was obviously not afraid of her in episodes like "The Algae's Always Greener".
- In "Tunnel of Glove", SpongeBob says he is afraid of clowns while he and Pearl are trapped in a dark amusement park ride. This is odd, because, not only was he never afraid of clowns before or since, he actually likes them in a previous episode "Krabby Land".
- In "Stuck on the Roof", SpongeBob suffers from a never-before-seen fear of heights, which presents a problem when he gets stuck on the Krusty Krab roof and can't get back down.
- In one episode of Teenage Mutant Ninja Turtles (1987), Leonardo was scared of snakes. They were his greatest fear in fact, until the end of the episode, by which he is no longer scared of them. It was never mentioned until that episode.