Finn the human hasn't had much of a backstory. Even in the cartoon the most we see of Baby Finn is him dancing around in his diaper or crying because he went "boom boom". In the opening story Finn and Jake come across what can only be described as a death tree, where our heroes find a book full of pictures of the duo's adventures. While Jake is rightfully freaked out that a death tree has possibly been stalking them, Finn thinks nothing of it. The book eventually sucks him in, and it's up to Jake to rescue his best friend from said book.
The story doesn't immediately grab me. In fact I skimmed through most of it. I assume through the white out scenes that Finn is traveling through his past lives, but it's frankly kind of boring. The remainder of the book is hit or miss, but I was a fan of the Hot Dog Princess story. When a ghost steals the youth of the Hot Dog people, it's up to Finn and a reluctant Jake to save them and restore their vitality. Of course there's more to it than that, but you're going to have to read it to find out.
Adventure Time was a fantastic series when it first aired. The first few seasons were charming and different. But then it got very repetitive and stale, and I stopped watching and reading it some time ago. Christopher Hastings is a great writer and captures the voices of the characters well, but overall the stories are bland. Ian McGinty is great at mimicking the look and feel of the show. When the Hot Dog people (food people?) have their youth sucked from them it is visually funny. He's great at visual gags which is the saving grace of this comic.