SO stands for Significant Other, apparently, making for a fucking horrific title.
The five year old narrator is sitting on the couch, watching TV with his father. His mother comes in. "Watching TV all by yourself?" That's weird, he thinks. Dad gets up and walks out of the room, and the boy plays cards with Mom. Then his father comes back in. "Solitaire?"
That night, Mom makes dinner for two. Dad brings home a couple of meals from the bento shop.
Neither can see the other. Both claim the other died in a train accident. Only the boy can see them both.
I couldn't help but get a serious Jojo vibe off this - Otsu Ichi is not only on record as a huge fan, he wrote the fucking awesome Part 4 novel. Something about the uncanny situation here really showed the influence.
Unfortunately, it also showed up Otsu Ichi's one real weakness - he occasionally fails to come up with endings worthy of the rest of the story. This tends to lead to him trying for a dramatic twist, unsupported by the story - as in Goth's Dog - or one that simply feels like a lame ass cop out. Wish someone had had the sense to cut the last two pages.