The story is so straightforward, and yet the complexities multiply. The documentary details a relationship so unexpected and curious and unfathomable. It sounds ridiculous, but this masterpiece shows how one man (Craig Foster) manages to establish a genuine friendship — a kind of love affair, really — with an octopus in the waters of a kelp forest at the tip of Southern Africa. At first, Foster is just as astonished as the viewer. After all, he didn’t venture into the water seeking a new friend. Who would?