- SpongeBob SquarePants
- Mystery the Seahorse (debut)
- Mr. Krabs
- Squidward Tentacles
- Patrick Star
- Wobbles (blue)
- Krusty Krab
- Old Man Jenkins
- Tina Fran
- Nathiel Waters
- Martha Smith
- Nancy Suzy Fish
- Sadie Rechid (cameo)
- Vera Fishbowl (cameo)
- Susie Fishbowl (cameo)
- Jimmy Gus (cameo)
It is Springtime in Bikini Bottom, and Patrick is emerging from his rock and storing away his Winter "coat" (sweater), burying it like a dog bone. SpongeBob, meanwhile, proclaims that he has "Spring Fever", and greets the flowers in his yard. When Squidward steps out of his house, SpongeBob asks him if he has said hello to the flowers as well. Squidward does so sarcastically, and the flowers hiss at him, causing him to run inside, terrified. SpongeBob concludes that Squidward has "Hay Fever" and, as an act of kindness, decides to plant him some hypoallergenic flowers. He does so, and the moment he takes his eyes off the flower to grasp the watering can, he discovers it's been eaten. Surprised, he plants another one, which meets the same fate as he again reaches for the watering can. Determined, he plants another one and doesn't take his eyes off of it for 3 days. Patrick says Hi to SpongeBob, and SpongeBob looks at him and says the same thing. But after he looks at the flower again, it gets eaten. SpongeBob then discovers that the culprit is a green seahorse. SpongeBob is fascinated and imagines him and the horse together riding, and decides to tame her. Later, as he watches the seahorse, it is lured toward him his floral bookmark, which it eats. They become friends and begin to ride around and do things together. As Squidward is biking to work the next morning, SpongeBob races past him on his seahorse, which he has named "Mystery". Squidward then rides off a cliff and causes a nuclear explosion when he hits the ground. When SpongeBob reaches work, he leaves Mystery outside at the bike post. Two fish approach Mystery and mistake her for a kiddy ride. One suggests the other to try it out, and when he inserts a coin into her "slot", Mystery kicks him into the distance, and he blows up on impact. The other fish runs away screaming, causing Mr. Krabs to come out and see what the trouble is. He sees Mystery and mistakes her for a monster; SpongeBob tells him she is his pet, but even though she is harmless to SpongeBob and not the others, he is ordered to get rid of it and if he disreguards his order, he will be fired for not cooperateing with him.
SpongeBob attempts to hide her in a cupboard in the kitchen until Patrick finishes building a stable. SpongeBob offers Mystery a Krabby Patty, which the horse begins craving after eating. After she craves, Mr. Krabs overhears this noice, and angrliy comes in and confronts SpongeBob, threatening to fire him if he sees Mystery still here, but, however, he dosen't see her and leaves. After narrowly avoiding being caught by Mr. Krabs, SpongeBob realizes he has to be more careful. Squidward catches Mystery after an order goes missing, and Mystery goes on an eating frenzy while a crowd of customers became angry. Mr. Krabs finally catches SpongeBob nursing a sick Mystery in the kitchen, who is suffering from a stomach ache, now knowing that he had broken the rules. Mr. Krabs then becomes angry with SpongeBob and tells him that he plans to get rid of both, Mystery and SpongeBob. SpongeBob is responsible for breaking and disreaguarding the rule, but he pleads for mercy, explaining that Mystery is his friend and that he loves her.
Mr. Krabs, feeling sympathy for SpongeBob, tells him a story of his favorite dollar and how he eventually spent it for a soda on a hot day. He tells SpongeBob that Mystery needs to be freed because the indoors is not a place for a wild animal, and Spongebob consents and takes the horse out the front door where he painfully asks her leave. He cries as she leaves without hesitation. Mr. Krabs comforts SpongeBob until Squidward shows him an empty safe. Realizing that Mystery ate the Krusty Krab's finances, SpongeBob and Mr. Krabs chase after her. The show ends with Patrick trying to get into a hat store, but the wooden plank nailed to his head blocks his way.
- Music to Drive By - Montage of Spongebob and Mystery.
- Popcorn - Title card
- Dramatic Cue D- SpongeBob sees Flower bitten again
- Magical Effect E- SpongeBob sees a seahorse
- Death Trap- "HELP! Kiddie Ride on the Loose!"
- Hawaiian Link B- "I knew I should have just named her Debbie."
- Dramatic Cue E- SpongeBob thinks the Krusty Krab is haunted
- Comic Walk- Squidward explains what he and SpongeBob do at work
- Danger Zone- SpongeBob notices that Mystery ate his hat, his spatula, all the Krabby Patties, the stove, and Old Man Jenkins
- Dramatic Impact 2- Mr. Krabs sees SpongeBob with Mystery
- Fates- "No Mr. Krabs! Please, don't make me give up Mystery!? I know you think she's just a horse, but she's more than that, I tell you. She listens to me. She understands everything I say and I understand her! She's my best friend!"
- Tympup A- "Hey, Who left this Bowl of Onions Here?"
- Unknown Tracks 2 - "GET THAT HORSE!"
- Mystery's nickname, "Sir Eats-A-Lot" is a reference to Sir Lancelot, one of the most famous knights from the tales of King Arthur of Camelot. This Name also could be a refrence to Rapper Sir-Mix-a-Lot
- When a fish complains "Where's our food?" with another saying, "I'm so hungry!", it references the live-action film Animal Farm, where farm animals complain of hunger like the fish customers.
- Mr. Krabs also yelled "scaring away me customers!" in "Imitation Krabs".
- The "Three Days Later" time card has the same pattern as the title card for "Squidville."
- The "Twelve Seconds Later" time card is also similar to the title card for "Help Wanted," except it is a different shade of orange.
- Just like in Jellyfishing, Squidward fell off a cliff and exploded once he landed.
- Patrick removed his winter sweater to begin his spring, but in "Snowball Effect ", he is not wearing a sweater despite the fact that it is winter. He is also not wearing one in "Survival of the Idiots " or "Christmas Who".
- First time the episode airs on the first day of Spring.
- Among the things eaten by Mystery is SpongeBob's hat, but in a later scene, where he attempting to give Mystery a bicarbonate soda, his hat is whole again.
- At one point, when SpongeBob lets Mystery eat his floral bookmark, he breifly has a mustache. For the rest of the shots, the mustache disappears.
- Frank Welker is erroneously credited as Tom Welker.
- In one scene, SpongeBob is shown holding the bag of hypo-allergenic flowers, but a few seconds later, the bag disappears.
- The captions, "(all yelling)" is shown on the bottom of the screen in the eastern version, but a little bit slow.
- Mr. Krab's narration about his dollar is still similar to Spongebob when they lose their beloved ones. Yet, it is unrelated to what he forces Spongebob to get rid of Mystery. This contradicts his advice.
- Look closely at the customer sitting near Mystery while Squidward is talking to SpongeBob you'll see the customer lips almost sync up to what Squidward is saying.
- In this episode, Squidward does not like flowers, but in other episodes he loves them.
- When Spongebob and Mystery were dancing, on TurboNick, a song from Nickelodeon's old show "The Naked Brothers Band" can be heard for an unknown reason.