Greta and Jonas discover a sack containing two drowned kittens in a lake; Greta's father, vet Philip, saves one named Goldfisch, prompting Greta to find the "cat murderer." Alongside Jonas and Lilie, she investigates, initially suspecting Mr. Schott, who has many cats, and plumber Mr. Akcay, while Philip urges caution. Their discovery of an old toilet chain and lead weights, used to sink the sack, eventually leads them to the junk dealer/plumber, the true culprit. Confronted, the man tries to escape but falls into the water himself, experiencing the terror of drowning before being rescued. Although he never reclaims them, Goldfisch is reunited with her mother, and both, along with other rescued cats, find loving new homes with Philip's family and Ms. Niethammer, ensuring a happy ending.