Oswald and Oliver Deuce (Brian and Eric Deacon) are brothers, separated conjoined twins, who are both zoologists. Their wives are both killed in a car crash. The driver of the car, Alba Bewick (Andréa Férreol), collides with a swan escaped from the zoo where the brothers work. As a result of the accident, one of Alba's legs is amputated.

The grieving brothers become obsessed with decomposition as evolution's logical complement, and begin exploring, by means of time-lapse photography, the process of decay of life forms of increasing complexity (while they watch, obsessively, the David Attenborough TV series, "Life on Earth"). As their experiments require more animals, they become involved in a shady scheme for procuring animal corpses from the zoo, a process involving a prostitute / teller of erotic tales who is sexually obsessed with black-and-white animals.

Alba, now with one leg, becomes obsessed with symmetry. She takes both Oswald and Oliver as lovers, becomes pregnant, and bears twins. She is persuaded by a Vermeer-obsessed aesthete veterinary surgeon to let him amputate her second leg. She decides to commit suicide and plans to have the twins film what happens to her body after death. When her family prevents them from taking her, Oswald and Oliver instead set up their time-lapse photography equipment and kill themselves, choosing to decompose together.