There isn’t much to the story in this book. Misaka and Touma get a room at a hotel to hide from the rain, and end up taking a bath together. It also involves Misaka hugging a cat for ultimate moe-ness.

As far as I can tell, the stories doesn’t add up, so I can only assume that each book should be viewed as an alternative story using the same settings, i.e. the Toaru universe and a Misaka x Touma pairing. The 4th part is a continuation of the 3rd part, since the title pretty much means “What happened after Sasoikata”. Also, the next part of the story will probably be a continuation of this, for obvious reasons. My interpretation is that the alternative stories are part 1, 3-4, 5-6, this being the 5th part, and 6 will be the next book to be released. The second part could go with any of the stories, since it takes place in Misaka’s imagination before they have sex for the first time in one of the three stories. When I refer to a book as a number, I use the chronological order in which they were released. See the link.
… Sometimes I feel like I spend way too much time thinking about things like this…