Protagonist/main character would be the kid in the middle.
(He would represent the player AKA you from the first game)
The Plot:
The kid and his family move into a new suburb (resembling the one from the game) due to wanting a new home or something.
After making new friends and going around suburbia, he starts hearing rumours and stories of some crazy man who's been doing strange stuff and often wacky.
Later, he first meets said man who goes by the name "Crazy Dave" who explains that he's been doing experiments involving plants making them into sentient living creations.
The two would slowly form a friendship overtime and bond with Dave himself being like a father towards the kid due to the child not having a dad.
(Either moved somewhere or unknown reasons why he's never seen)
Way later on, they start hearing news reports of undead people coming from graves in an old cemetery and causing havoc and bit of antics, this is none other than the work and schemes of Dr. Zomboss, an evil mastermind who plans to take over and put zombies above humans as the new dominant species.
So, the kid and Dave would be creating plants (Some classic recognizable ones and even new originals) to take on the zombie army, not only to protect each other but everyone including the kid's family and the suburbs too.