The movie is based on J.R. Ackerley's memoir by the same name. So the film is essentially an animated monologue (with Christopher Plummer providing the voice) by Ackerley about his life with Tulip. What makes this interesting is that Ackerley is a pretty strange guy. I mean, the dry wit and vaguely vicious contempt towards the world, you sort of expect, but what makes this movie in particular so intriguing is the sexual aspect. A large portion of the movie is about Ackerley's attempts to get Tulip pregnant (not personally, but to arrange and facilitate the process). This turns out to be highly a somewhat complicated process, and comes to involve more and more lascivious observations about both dogs and people. It's not exactly perverse, but it is a bit surprising to hear such candid - not to mention eloquent - descriptions of a dog's vagina. It's also kind of great in a liberating, celebratory sort of way.
The movie is ultimately not that great - there are plenty of nice observations about dogs and companionship and life, and the animation is nice, but it does drag a bit, and there's not really much narrative to latch on to, and the charm sort of wears off. Still, not an unenjoyable viewing experience.