There's a crime in the park and the evidence points to 101 Dalmatian Street, so Dylan turns detective. After words, Dylan thinks he can handle the investigation alone, refusing help from his little bro Diesel who has super sniffing skills.
The Episode begins with the Camden residents in the park, just as all the Dalmatians arrive. Dylan is in a rush as Godfrey the Guide Dog is hosting the tenth annual Nose Off, a competition where contestants need to identify a hidden item based solely on scent, and Dylan has been studying for it. On the way there, they collide in to a picnic which Clarissa is having with Arabella and Prunella, as well as Constantin the Cat (who is meditating) and a hairless cat.
Dylan is able to arrive and sign in to the contest in time, as well as Diesel, but Dylan notes that Diesel has not studied for it. Fergus is also part of the contest, but gets disqualified when Dolly points out that Big Fee is hiding behind the stand and miming what the hidden item is to Fergus. Soon, it is down to Dylan, Diesel and Spencer the Sausage Dog. Spencer mistakes the scent of a sausage for bacon and is out of the competition, with the Dimitri Trio laughing about how a Sausage Dog got the sausage wrong.
Godfrey then, for the tiebreaker, asks Dylan and Diesel to identify the most recent flowers a Bee has pollinated, with Diesel getting it correct and winning the contest, causing Dolly to tease an angry Dylan about this.
The next day, Pearl the police horse arrives at the house to tell the Dalmatians that the park has been vandalized, with some one digging up the flower beds and painting paw prints over the Horse Statue. Since the Dalmatians are suspect (such as Diesel's habit of digging, and Da Vinci's art) they are banned from the park, until Pearl can complete her investigation.
Dylan decides to investigate too, but he can not get an exact scent as all the animals' scents mix in to each other. Dolly suggests that Dylan ask Diesel for help, with his more precise nose, but Dylan refuses, likely still bitter about the contest. He then suspects some one is watching them, with Dolly thinking he is being paranoid, but an unseen observer really is recording them from afar.
Dylan first questions the Canal Crew. The three explain they were raiding trash cans at Primrose Hill at the time of the crime, and can back it up by the fact they woke the residents doing so! Dylan then questions Clarissa the Corgi, but she presents a newspaper, showing that she was camped out side Buckingham Palace all night, waiting for the Royal arrival. Diesel arrives and notes he can not smell park flowers on Clarissa, and Dolly repeats to Dylan that Diesel would be helpful to his investigation, but Dylan still refuses.
Dolly then suggests it being a feline, but Dylan notes that his cat allergies would have picked it up. Dolly then suggests asking where the paint trail leads. Dylan goes back to the Canal Crew, and they explain they found some paint cans in Spencer's trash. Dylan suspects Spencer is getting revenge for the other pups laughing at him, but Spencer reveals that he was in fact painting Portia's doghouse, the night of the crime.
Dylan then begins to follow Godfrey, but Diesel picks up from scent that Godfrey was visiting his human's mother in Southend, during the time of the crime. Dylan begrudgingly accepts Diesel's help.
Dylan then asks Diesel "Who" was in the park that night, but Diesel can not pick up any one's scent. This confuses Dylan, before he then suspects some thing. He and Dolly search Diesel's basket and find it surrounded by flower petals, leading them to think that Diesel is the culprit. Dolly, however, does not think Diesel is a criminal mastermind type. That night, Dylan has a stake out and follows Diesel to the park, where it is revealed that Diesel is sleep walking, as well as sleep digging. Diesel digs his way in to the park, where he is quickly arrested by Pearl.
At the police station, Dylan tries to explain about Diesel to Pearl, but Pearl notes that Diesel did still break the park ban. Dylan tries to send a message on the World Wide Woof to help rescue Diesel, but his messages get mixed up to the point where a Barking Chain happens, so when it arrives to Dolly, it is about shaving a weasel.
In the station, Diesel sees two other dogs, as well as Snowball, who is in the station as Pearl got caught up in her leash (Snowball's human oblivious to where she is).
Dylan returns to the house and wakes Dolly up, telling her that Diesel is in prison. The two then examine the flower petals in his basket, and realize they are not the same flowers as the ones from the flower bed that was vandalized. Dylan then gathers the other Dalmatians together to break Diesel out.
While Dylan and the other pups try using the World Wide Woof again, to distract Pearl, Dolly and Da Vinci (covered in soot from Doug's helmet) enter the police station from the skylight. Da Vinci then paints on the wall behind Diesel's cage, making it look like Diesel has dug his way out.
With Pearl searching out side (the other pups, disguised as Diesel, keeping her distracted), Da Vinci uses her spray can to keep the door from closing, so she, Dolly and Diesel can escape through the front entrance. During this, they over hear Pearl claim a Cat reported that it was the Dalmatians. Unknown to the other Dalmatians, they are being recorded from afar. When Pearl realizes the hole in the wall is actually painted on, she accidentally trips on Snowball's leash, allowing the Pomerania to squeeze through the bars of her cell and be dragged back to her human, with her retracting leash.
Dolly explains to Dylan what she heard, and they suspect Constain is the cat in question, who reported them (as the pups did collide in to him, while he was meditating). Dolly takes the other pups home before Pearl comes round asking about what happened.
At Constantin's house, Dylan hides behind the meditation statue, and poses as a spirit, asking Constantin why he accused the Dalmatians. Constantin realizes it is Dylan and tells him he was not the cat who reported them, but suggests it may have been the hairless cat who did.
Dylan does not think the hairless cat is the one who committed it, since he and Diesel did not pick up any one's scent at the crime scene. Diesel points out howeve, he did pick up a rubber scent at the scene. When Dylan questions why Diesel did not tell him sooner, Diesel explains that Dylan asked him "who" he could smell, not "what".
Dylan follows Diesel to where the rubber scent leads and they find some rubber dog paws in the trash. Diesel picks up the hairless cat's scent on them, with Dylan realizing that them being hairless is the reason why his allergies did not pick them up.
It is then they are caught by Pearl, who plans to bring them in, but Dylan presents the rubber paws to her and ask they present the evidence at the statue first. Pearl allows them to when Dylan suggests that solving the case would win her "Police Horse of the Year".
In the park, Dylan shows the rubber paws match the prints on the statue, and Diesel is able to use his nose to locate the hairless cat who was watching them from the bushes. Dylan, Diesel and Pearl attempt to catch them, but the cat escapes in a limo.
Back at Dalmatian Street, The pups celebrate and Dylan and Pearl thank Diesel for his help.
Back with the hairless cat, whose name is revealed to be Cuddles, the audience learns they are in fact the cat of Hunter De Vil, who was the real mastermind behind the crime and is watching the footage that Cuddles had recorded of the Dalmatians.