Based on a four-part series of novellas included in the Blu-Rays of That Time I Got Reincarnated as a Slime's second season, Visions of Coleus is set early in the TV series' second season. Everything here takes place at a time before Rimuru's ascension to True Demon Lord—i.e., before the attack on Tempest and the subsequent retaliatory massacre of 20,000 men. This means that, at this time, Rimuru, while still powerful, is not the unstoppable force he will eventually become.

Honestly, this is a welcome return. There are forces in this story that Rimuru would be unable to beat in a fair fight—and even more that would push Rimuru and his friends to their limits. Thus, there is a fair bit of tension in the story as Rimuru must use stealth and tricks rather than raw power to uncover the mystery of what's going on in the Kingdom of Coleus.

The strongest point of this ONA is how it subtly forces Rimuru to acknowledge his past in a way he hasn't since he reincarnated in the fantasy world. When thinking up an alias for this stealth mission, he picks the one other name he is sure to respond to, Satoru—i.e., his name from his first life. Then, he meets Princess Zenobia, who can see the shape of a person's soul. While this mainly allows her to see if someone is good or evil, in Rimuru's case, she sees the man he was before his death on Earth. Thus, when he becomes the "Great Phantom Thief Satoru" for the sake of his mission, he chooses not to look like Shizu but rather like an idealized version of his original self. For the first time in years, he's back in his old body—and still trying to help others like he did back then.

This ONA also benefits from a reduced cast. While Souei and Benimaru have a role in the final act, the majority of the story revolves around Rimuru and a new collection of characters. Without having to give each member of the TV anime's expansive cast their moments to shine, we are rewarded with a much tighter story—even if it is one with some baked-in issues of its own.

While the general story is easy enough to follow, many viewers will not understand some parts of this ONA. This is because, at its heart, this ONA doesn't feel like it's for the watchers of the TV anime but rather for the readers of the original novels. Both Luminous Valentine and Violet are major characters in the events to come. Knowing their goals and motivations puts their actions here into a context that anime-only watchers will simply not have at this point—which can leave the story feeling oddly under-explained by the time the final credits roll.

Moreover, the inclusion of both Violet and Luminous Valentine cornered the story. It is already set in stone when Rimuru first encounters both characters. Thus, they can't meet in this ONA—meaning the story has several convenient contrivances to ensure they don't. Honestly, it feels more than a bit forced when this amounts to Rimuru, Souei, and Benimaru waiting patiently inside an opaque bubble (that Rimuru can break out of with ease) with nothing but a corpse for company for several minutes while Luminous Valentine and Violet go at it.

All in all, Visions of Coleus is a serviceable little That Time I Got Reincarnated as a Slime side story. While novel readers will get more out of it than anime-only watchers, this flashback to an earlier time in the greater story delivers a solid mystery with more than a few twists and turns. If you're already invested in That Time I Got Reincarnated as a Slime, there's no reason not to check this out.