As soon as Zenitsu learns (off-screen) that Nezuko is Tanjirou’s sister, he starts chasing her around their room. When Tanjirou tries to stop him, Zenitsu starts chasing him around too. Then Inosuke, who is ticked off from hunger, starts chasing them, leaving Nezuko out of the loop.
A fun time is had by all, but the next day the doctor declares the three lads have fully recovered, and a crow arrives with fresh destination: Mount Natagumo, with no further explanation except that there’s bound to be demons. With Nezuko secure in her box, the slayers set off for their first mission together.
Upon reaching the foot of the mountain, Zenitsu senses something creepy and chickens out. Tanjirou prepares to press on, but Inosuke rushes ahead of him; later Tanjirou thanks him for going with him, but Inosuke either didn’t do it for him, or he did and doesn’t realize it. He’s a very enigmatic fellow who is decidedly not brushed up on the social graces.
Once on the mountain, the two encounter a third demon slayer being carried off as if by some invisible threads. Eventually learn that he’s one of ten slayers sent to the mountain to deal with a demon infestation, but they’re all too low-ranked, and end up being turned into puppets, the strings connecting them to a family of spider demons.
Tanjirou and Inosuke sever the threads from the slayers, but neither that or killing the countless tiny spiders making the threads will solve the problem. They need to find the demons behind this, and that’s when Inosuke’s Beast Breathing Form Special Awareness comes in handy. In a very cool sequence his awareness stretches out across the forest until it locates the lead spider demoness, then travels back to Inosuke.
They now have her location, but her apparent son is standing on threads suspended high above the ground, just out of Inosuke’s vertical reach. The boy declares that no one will stop his family from living a happy life together, and that made me wonder: Why exactly are the Demon Slayers bothering with these guys? They’re secluded in the mountain forest, minding their own business. Their only victims seem to be intruders who couldn’t mind theirs.
I guess demons can’t be allowed to be left unchecked, lest they expand their territory and start threatening human settlements. The Demon Slayer Commander certainly seems resolved to eliminate the threat of them. When a crow arrives to report the lower-ranked slayers’ defeat, he prepares to send two Hashira slayers: Giyuu and Shinobu.
Combined with Zenitsu realizing Tanjirou took his beloved Nezuko into danger, that will eventually make it five slayers plus Nezuko against the five spider demons. They’ve just got to hold out and wait for the cavalry.