But.. it's the story behind the show that's rather upsetting...
Finn was a 13 year old boy who was extremely lonely: having no friends and being ignored by his mother, who was having a difficult time enough trying to support him working 2 jobs. His only friend was his loyal dog, Jake. A few years later, Jake died of old age. After Jake's passing, Finn got extremely depressed and attempted to commit suicide via overdose. Rather than dying, though, he fell into a coma and was stuck in the hospital. Doctors feared he would never wake up from the coma. Sadly, they seemed to be correct, as Finn would never wake up from his coma, stuck in a perpetual dream, a fantasy world Finn had always dreamed of, and told Jake about.
In this world, Finn and Jake went on crazy adventures to save princesses that were madly in love with Finn, Jake had a girlfriend named Lady Rainicorn, who was a rainbow colored unicorn who only spoke Korean (which led to only Jake understanding him). Finn, however, was in love with Princess Bubblegum, who ruled the Candy Kingdom, but she could never decide whether Jake was her friend or her lover, which served to be a constant source of frustration for Finn. His greatest rival was the Ice King, an evil king who ruled the Ice Kingdom, who was upset and depressed because he was forever alone, so he kidnapped princesses to "pleasure" him. The Ice King had an intense jealousy for Finn because the princesses loved him so much.
More and more characters came into the picture. More candy people, more talking inanimate objects, and more adventuring. Finn had went completely insane. It was no normal child's imagination at work. Finn would remain in a coma until his dying days, adventuring with his dead dog Jake, never aging, never changing. It was forever, Adventure Time.