When Ruiko proudly, flamboyantly shows Mikoto how much her kendama skills have improved (showing off like nunchaku) through Dream Poker, its to encourage her friend not to give up on the fad just because she had a bad first experience. Meanwhile, Kinuhata Saiai of ITEM (this arc takes place before the group was destroyed) has discovered that of the four female members she has the smallest bust, and is not okay with that.
Saiai and Mikoto, two young women overly self-conscious about their size, end up crossing paths at a street poker card street dealer. The one S-Class card in his stock happens to be the dream of a “long-lost but brilliant scientist” researching a certain matter near and dear to them. When the two learn it is called Bust Upper, they’re both on the cusp of starting an esper brawl when the dealer breaks them up and all the cards scatter.
Rather than fight, Mikoto and Saiai decide to each buy up half the cards and try them all out until they find the Bust Upper, which got lost in the pile. Yet neither end up dreaming with that card, only finding themselves in scenarios in which society and the world persecutes them due to their modest natural bust sizes. Both get so frustrated they end up waking up loudly ranting in the napping center, and are politely kicked out.
They continue searching the cards poolside (though not in swimsuits), and aside from an extremely cryptic, abstract dream involving a mysterious black-and-white-haired woman (of whom we certainly haven’t seen the last), they come up empty again. They part ways “ultra”-disappointed, but the Railgun and ITEM having enjoyed a truce over a shared lack of a burden (on their shoulders), and believing the dealer was simply telling tall tales.
However, the real Bust Upper card ended up snatched up by a crow while both were napping. When a cat startles the crow, it drops the card near the hand of a girl in the park with a modest bust of her own. The next day, as Rikou is showing Uiharu her new master pen-spinning skills (love her trend of only mastering useless skills), they unwittingly witness the results of Bust Upper, tying the episode up with a neat bow.
As far as Railgun filler goes, it rarely gets as filler-y as this, and takes the patently silly bust complex comedy about as far as it can go. But it’s nevertheless a fun time because it’s bursting (busting?) with Railgun charm straight out of the gate. It’s great to Mikoto enjoying some ordinary days before the next threat shows up, and especially nice to see ITEM during better days.
P.S. Considering how hard she worked the past two weeks, it’s understandable that Kuroko just wants to get some shut-eye when Mikoto, who is overly rested from all the napping, talks about her day. Kuroko doesn’t even react to the mention of Missy in a revealing outfit!