To get closer to his ultimate goal, Mutta must first move further away from it, as the latest step in his training to reach the Moon sees him move deep underwater for NEEMO training.
As part of a group of trainee astronauts, Mutta is thrilled to find himself paired up with Kenji alongside a couple of more experienced individuals, both of whom seem to be rather dismissive of the newcomers. What Mutta doesn't know is that, although Kenji is indeed his team-mate for this part of his training, he's also ultimately his rival, as only one of the trainees will be picked from each team at the end of this stage of their training.
While Mutta jumps into the new experiences that come with training under the sea, approximating lunar conditions as closely as you could hope on the Earth (albeit with some rather marked dangers that you wouldn't find on the Moon - barracuda, for example), Hibito is facing challenges of his own as he co-opts Lowry into helping him with his rehabilitation. With a member of the medical staff also on-board to help him out in secret, perhaps he too is moving closer to the Moon himself.
As episodes of Space Brothers go, this was another of those instalments that works more towards setting the scene for episodes to come rather than delivering anything particularly unique in its own right - thus, we see both Mutta and Hibito settling down into new routines, with clues as to the kind of struggles that might be ahead for both of them (and Kenji, judging by the episode preview for that matter). It certainly feels like the show is currently positioned to offer up some of the kind of content that showcases the series at its strongest, so hopefully that proves to be the case.