A depressed 60 year old man who's wife has recently died lives alone and in sorrow. His days blur in to one long miserable senior life. Until one day he finds a mouse in his basement. Terrified of them he sets a mouse trap. While cooking in the kitchen later on he hears the trap snap from the basement.
He walks downstairs to find the mouse caught in the trap, with the metal bar snapped across its back crushing its spine. The mouse is still alive, wheezing and making mouse noises as it gasps for its last breathe. The man watches the mouse suffer and a he begins to grin. He pulls up a chair to continue and watch the mouse suffer until it dies. However as the mouse stops to make noises he realizes he wants to watch it suffer more, he prys the trap back and allows the half dead mouse to breath more easy. The mouse finally dies and the old man realizes how much pleasure he got from watching the mouse die. He drives to the store and buys a hampster.
The old man brings the hamster home and kills it, he gets happier. He then buys a dog from a rescue shelter, brings it home, kills it. The man gets happier. In the back of his mind he realizes if killing animals makes him so happy, how happy would he be if he killed a person.
His desperation to be happy grows as he chooses his next victim...he wouldnt be strong enough to pick a full grown adult, he's too weak to capture one...there are more than 20 children on his street, and he can pick anyone he wants.
All these happen through out the first half of the movie. After killing a child and watching the family grieve(he even goes the childs funeral and sits with a wide grin on his face) the man realizes his lust to be happpy can not continue. He struggles to try and find new ways to be happy, as all he can think about is being happy again. The objective is to make the viewer feel sad for the old man because the only thing that makes him happy, he can no longer do.