Alice is a young, bright-eyed, hopeful girl, but one who has her doubts about the validity of this whole "Santa Claus" thing. Bob, who may be her father, or an older brother, or a mentor of some sort, dismisses such thoughts by assuring Alice that yes, Santa is real. The kicker, however, is that Bob is making stuff up and is probably just trying to get Alice to go to sleep. He doesn't actually believe in Santa! He won't actually come out and say this though...
...Until the real Santa comes around and gives Bob the shock of a lifetime by proving that yes, he is real after all!
The Santa's Existence Clause is pretty much the story outlined above: One (or more) character who doesn't believe in Santa trying to convince another of Santa's existence, and it turns out that they were right! A lot of times, Santa won't actually present himself to Bob, but only to the viewer. Just try not to melt the snow in the process.
All in all, this is basically an amalgam of several smaller tropes all being played straight just to subvert each other. Many stories like to see-saw, subverting and inverting the idea that Santa is real over and over again until the very last page.
Many of these stories start on the cynical side of the Sliding Scale of Idealism vs. Cynicism, but end up on the ideal side (due to Bob's lack of belief in Santa being proven wrong)
Note that this trope usually doesn't apply if Santa Claus is the main character. Usually he's a one-off guest in a continuing series or maybe just in as a cameo, almost always in the Christmas Episode.
Contrast with Yes, Virginia, where Bob (or a bunch of Bobs) is trying to tell Alice that Santa doesn't exist. However, there is some overlap in that Santa is usually shown to be real in such cases, as well.
This sort of plot tends to have a bit of Fridge Logic in that Santa turns out to actually be real, which makes it strange that these adults are disbelieving (or actively denying) his existence. Didn't they notice the millions of toys showing up every year that nobody seems to have purchased? If they grew up in a world where toys magically appear every Christmas Eve, how did they come to collectively reject the existence of the delivery man? Often the answer is another Fridge Logic in that it doesn't seem to happen; even though Santa is apparently making deliveries all over the world, no one ever gets any extras unless it's a plot point! Likewise, if the series has already firmly established that magic and the supernatural exist, there can be a heavy amount of Arbitrary Skepticism involved. Compare and contrast Santa Ambiguity, where it remains unknown.
- A commercial for M&M's has Yellow wondering if Santa likes M&Ms, with an exasperated Red grumbling that he wouldn't know because he's "Never met the guy!" Suddenly, Santa comes barreling down the chimney, causing Red to exclaim in confused excitement "He does exist?!"... followed by Santa realizing the same thing about the M's. They both pass out, leaving a bewildered Yellow just standing there.
- Discussed in episode 2 of season 2 of Love Live!. When the girls arrive at Maki's family home in the mountains for a song brainstorming session, the latter asks them not to use the chimney, partly because it's still too warm outside to use it, and partly because her father told her Santa won't visit if the chimney isn't kept clean. She then shows some writing and drawings from "Santa" inside the chimney, prompting Nico to laugh at her for still believing in Santa. However, the other girls don't want Maki's belief to be shattered, so they try to keep Nico quiet about it.
- In the Clerks Christmas special, Randal is asked by a chubby, bearded man to find him some dwarf porn. Randal is about to refuse, knowing that this guy doesn't have an RST rental account... except that "Mr Nicholas" does, and his address is an apparently unknown apartment between the two stores. Randal finds out the apartment leads to Santa's secondary toy-making workshop, and that the old guy is real after all (the dwarf porn is for the elves btw, so relax)! He gets some weebles and tries to score a Mötley Crüe denim jacket to win a bet against Dante... But later, Randal wakes up in the snow outside, and his head hurts. Turns out he was cold-clocked by another "block o' stores" employee, and that the apartment has been vacant for months, and the whole things was All Just a Dream... Until Jay and Silent Bob come along, who trade him a blunt and a weeble... for the Motley Crue denim jacket Jay is now wearing.
- A Justice League of America comic had Plastic Man telling Woozie Winks' nephew about how Santa was indeed real, and even a member of the JLA. His story is obviously bogus, as he relates the tale of how Santa helped beat Neron (One of DCs innumerable demons pretending to be Satan) while having the powers of flight and heat vision. Just as the little snot begins lampshading every detail ("If you guys were in Hell, why wasn't the fire hurting Martian Manhunter?"), Santa(!) comes flying by, and much to Plastic Man's shock, uses heat vision to burn "Merry Christmas!" into a nearby snowbank! The nephew goes to sleep, knowing Santa is near... Plastic Man meanwhile goes to relate this news to Woozie, completely in shock over it all. Turns out "Santa" was just Martian Manhunter, who, along with Green Lantern, was in the area and heard Plas's whole story because his signal device had been left on — but then in the last panel, the real Santa makes an appearance, saying "Heat vision? The imagination of some people!" Apparently, the part about being a member of the JLA is true. He also breaks into Apokalips every year to give Darkseid a lump of coal, and was killed by Lobo at one point. (like that'll stick)
- Played With and Inverted in Mickey's Once Upon a Christmas. After a talk with Pete, Max starts to doubt Santa's existence, and becomes even more convinced when he catches Goofy pretending to be him for some poor neighbor kids. The thing is, Goofy really does believe in Santa, and by the end, it's Max who dresses up to restore his father's faith. Naturally, Santa actually flies by near the end, to their delight (and Pete's shock).
- In Ernest Saves Christmas Santa winds up in jail, though it's obvious it's him from the get-go: his fingerprints show up as snowflakes, he's able to bring out the good in people, knows things nobody else could, etc... but the rest of the adults don't want to believe. Except for Ernest P. Worrel, of course.
- Happens to Scott Calvin in The Santa Clause, even though he spends the better part of his visit from St. Nick and successive trip to the North Pole trying to convince himself it's a joke and that he's deluded, respectively, completely blowing the false pretense he had made of believing in Santa.
- Subverted in Hogfather: Susan's "assurance" to Twyla that the Disc's version of Santa exists is a deeply sarcastic twist on "Yes Virginia, there IS a Santa Claus", but Susan knows the Hogfather exists - she'd just prefer to live in a world where he didn't. And then she gets surprised/annoyed when someone else comes down the chimney...
- Both averted and played straight in A Christmas Story as Santa's appearance is clearly a dream and the little boy knows it, but he chooses to believe in Santa regardless.
- Doctor Who: "Last Christmas" zigzags this. Santa Claus is explicitly stated to be a dream construct trying to save the characters, who have been trapped in dreams by the Monster of the Week. He, however, admits that he exists in their belief. Then, at the end, after everyone has woken up and the Doctor and Clara have taken off for more adventures, there's a shot of a tangerine, which Santa had earlier stated to be his "signature gift", on Clara's windowsill, adding a Maybe Magic, Maybe Mundane twist. And, of course, this isn't even mentioning that way back in "A Christmas Carol", the Eleventh Doctor claimed to have met Santa personally, and that he prefers to be called "Jeff".
Rose: "Look at you, dancing around like you're Father Christmas!"Doctor: "Who says I'm not - red bicycle when you were twelve."
- The show also suggests that the Doctor himself may be Santa Claus.
- An early episode of Home Improvement has Mark questioning his belief in Santa after Randy and Brad cruelly tell him that Santa died right before he was born. The end of the episode has Santa showing up at the Taylor house to bestow early presents on everyone and restore Mark's faith. After Kris Kringle leaves Tim comments on what a great guy their neighbor Wilson is to do something like that...only for Jill to point out said next-door neighbor standing at the fence waving to the family. Scene cut then to Mark standing at the front door and looking up at the sky in wonder.
- The Murdoch Mysteries Christmas Episode "A Merry Murdoch Christmas" features a subplot about an old man with a white beard who claims to be Kris Kringle, and everyone's impatience with Constable Jackson who is very close to believing him. At the end of the episode, when the constabulary are handing presents to orphans, a whole stack of new presents shows up without anyone knowing where they came from, and Jackson and Dr Ogden briefly see Kringle in the crowd in full 19th century Santa outfit before he vanishes.
- In Smallville, Season 5, Ep 9, Clark finds a drunk man dressed as Santa who is about to jump off of a building because Christmas spirit is dead. Clark, in his typical fashion, convinces the man otherwise. Later on, Chloe (who is expected to deliver a roomful of toys to needy children), receives a visit from the same Santa, who offers to help deliver, and she eventually agrees. She turns around for a moment, but when she immediately turns back around, the Santa and all of the presents have disappeared. She later speculates that maybe it was really Santa Claus.
- Played with on Qi, for their Season "K" Christmas episode, "Kris Kringle", when Stephen Fry asked "Why is Santa off the Rich List?" Brendan O'Carroll asked, "Could it be because (stage whisper) he may not be real?" The klaxon sounded and the words "HE ISN'T REAL" flashed on the screen. Phil Jupitus turned to the screen and fell to his knees in seeming despair, prompting Stephen Fry to assure him, "That got a klaxon so it can't be right!" and give him a hug. The Rich List, for the record, is an annual list assembled by Forbes Magazine of the richest fictional characters (that year's winners in ascending order were Tony Stark, Jed Clampett, Carlisle Cullen, Flintheart Glomgold and Smaug). Santa Claus had been on the Rich List before, but Forbes' editors decided that he didn't qualify because, in their opinion, he wasn't fictitious.
- Played straight by The Cult of Scratchwood: Matt spends most of the Christmas Special The Night Before Christmas trying to explain to Derek that Santa Claus is not real, and struggling to explain why humans pretend he is (the closest Derek comes to understanding is when he assumes it is "a tool for propaganda"). After Derek has already left, Matt sees Santa's flying sleigh whizzing past his window. He blinks and goes get another drink.
- Futurama has several episodes featuring Santa Claus... except instead of being a mythical jolly old man, he's an evil death machine that's frighteningly real. The explanation is that they tried to build a real one, but it's standards were set too high and it has no sense of proportion.
- Jackie Chan Adventures: When Jade caught Jackie taking a tooth from her pillow, it not only made her stop believing in Tooth Fairy but also in Santa Claus. She learned otherwise about Santa when they had to save him from Daolong Wong. (Note: While Uncle already knew — or at least believed — Santa was real, he doesn't believe the same about the tooth fairy)
- On Rugrats Chaz decides to be Santa for Christmas, while Drew hired an actor. After Chaz reveals his identity to Chuckie, Santa soon arrives at the door to hand out presents. The actor later calls to say he can't make it as Santa rides off in his sleigh and the adults wonder who he really was.
- Played with in The Real Ghostbusters when the Ghostbusters don't believe that the three ghosts of Christmas are real until after they've put them in the containment unit. At the end of the episode, once everything's been put back to normal, the Ghostbusters have no issues accepting the idea that Santa is real.