It's as you would've guessed from the cover. A slice of life kind of show about the main character getting into trouble with detective stuff long after his partner is crime has perished. So far, the plot is something that would be no different whether Siesta was dead or not. Two high schoolers solving mysteries and dealing with bad guys along the way with the heroine falling for the MC along the way. In a sense it's a convoluted form of a romcom.
All of this is ignoring one thing that really bugs me. The whole show makes Siesta out to be the main character, the main heroine of the anime and yet is explicitly states that she is already dead, heck even the title is telling you, and yet most of the character's development is based around her and not themselves. It makes it seem like their all just another aspect of Siesta and not their own individual people.
Well if you take all this out and just watch it regardless. The show itself is surprisingly rather enjoyable. The things that the duo do together can be fun to watch. It's just that the show really wasn't what I was expecting.
