Euthanizer tells the story of a mechanic who euthanises sick and old pets as a side job. Despite the grim work, he has a soft spot for the animals. Trouble stirs when the owner of a dog he was supposed to end realises that the canine is still alive. The only innocent party in the drama that ensues is the dog itself.

Veijo Haukka, who has mended his ways during his murder sentence, has to face against the offspring of his victims. At the same time closeness of death threatens his relationship.