Since he doesn't have the resources like Fitzgerald did to try and destroy the city entirely, he uses his cunning mind which is on par with Dazai's own and pits the Port Mafia against the Armed Detective Agency. Fyodor, the 'demon', is on the move to attempt to obtain the book for his own purposes. The next set is the major arc of the season, the House of Rats arc which spans episodes 8-12.

These 'fillers' (again, they were in the manga, so not really fillers) between episode 4 and episode 8 (where the House of Rats arc really starts) are rushed and though were fun to watch, were severely lacking to their manga counterparts. We see what Fitzgerald is up to, and there is an episode involving Kyouka and Demon Snow.as well as Atsushi and the Headmaster in that same episode. The next episode is a prelude to the House of Rats arc (the major 'arc' of the season), and then it goes into what seem to be filler of wrap-ups after the incident with the Guild. This was the best arc out of all of them, a solid 10/10. It revolved around mostly Chuuya as he searches for his relation to a creature of chaos/destruction which leads him to running into Dazai and the mafia, and is the story of how he joins the Port Mafia. The first arc was probably the best this season had to offer, and spanned episodes 1-3. There are two major arcs (with a few 'fillers' in between I put that in quotes because though they seem to be fillers, they were still in the manga). I highly suggest reading the manga of this arc (the first three episodes aren't in the manga but instead have a light novel as the source material starting from episode 4 is where the manga picks it up), because it's much better. First three episodes aside, this season can be described in one word: Disappointing.

Bungo stray dogs