Cat Soup is an extremely difficult film to describe, due to its awesome surrealism and almost nonexistant dialogue. It is more a work of art than an anime. There are many underlying themes, such as the transcience of man's existence, but it can be enjoyed without understanding any of them. The plot, though simple, isn't necessarily understandable without being explained. The main character, a cat named Nyatto, embarks upon a journey to save his sister's soul, which was ripped in two when Nyatto tried to save her from Death. She trails after him, brain-dead. They encounter many brilliant, mind-bending situations, beginning with a disturbing magic show.
 
(Source: Anime News Network)

Everything would have been perfect in the best of possible worlds if Death had not decided to come and pick up the hero's little sister. Despite being a kitten, he does not intend to let himself be carried away by this sinister character. Between fascination for the absurd, and immoderate taste for a sweet wickedness, Nekojiru stages Nyato and Nyata, who under their airs of cute little kittens, will set fire to the whole planet. 
 
(Source: IMHO, translated) 
 
Note: 
-Originally serialized in Monthly Manga Garo for 31 chapters, starting with the 1990-06 and ending with the 1995-10 issue. 
-Incomplete 2-volume tankoubon (23 chs) published by Seirindo in 1992 and 1995. 
-Complete 3-volume tankoubon published by Bungeishunjuu in 1998 and 1999. This tankoubon collects all chapters with the exception of "□nugoroshi no Maki" (□ぬごろしの巻). It additionally includes 3 newly drawn chapters (one in each volume) as well as the One Shot "Nyango-tan" (にゃんごたん) by Nekojiru-y, originally published in COMIC Gon No. 3 (November 1998). 
-The tankoubon releases change the order of the chapters. The credits on this entry are according to this ordering. 
-The chapter "Neko Saiban no Maki" (ねこさいばんの巻) was published in English as "kitty court drama" in the anthology "Sake Jock" by Fantagraphics Books.

Based on the comics by the late Nekojiru. Contains 23 animated shorts aired TV Asahi Network's show "Bokusho Mondai Boss Chara Ou." and 4 unbroadcast episodes. 
(Source: AniDB)

Nyako and her younger brother, Nyatta, are two cute cats living their daily lives in a realistic world riddled with violence and madness. A slapstick black-humor manga that became the posthumous work of the author, who passed away suddenly in 1998.
 
(Source: Shueisha, translated)

In an apartment that has become chaotic, with mysterious residents and inexplicable ghosts, the cute but mischievous and sharp-tongued Nyako and Nyatta are on a wild spree!