A neat hack with an interesting story about a Ninji trying to figure out what happened to his friends leading him through a series of tests and experiments by one alien Zookeeper. The text is all in message boxes rather than unskippable cutscenes but the story is told through the gameplay anyways. The dialogue does have a few spelling and grammar errors.
This hack is now 10 years old, and is probably one of the earliest existing examples of hacks using ExGFX. There are some little issues like cutoff which you'll only notice with a solid squinting stare, and rather big issues like glitched graphics in the credits, when the female Ninji throws you a garbled coin, and the lava splash graphics. One thing that became an issue for me gameplay-wise because of graphics was how hard it was to discern the walls from the background in one of the Magikoopa-infested laboratory levels.
Level design here becomes quite tough really fast when you get into the laboratory. Subconian Spy is riddled with slowdown and overloaded with sprites and has one jump that's a pain in the ass to get through unless you kill enough sprites to make comfortable way for running speed where the slowdown won't throw off your timing and I think this level could've definitely taken advantage of the SA-1 which found its way into SMW hacking only recently. As it is in the hack, it's the hardest level by far.
Ninji is always one tile high and has one hit point and this does impact gameplay in a huge way: it allows more comfortable use of very narrow corridors, and it adds a sense of urgency to be cautious.
I've got a soft spot in my heart for this hack as it's the first SMW hack I've ever played and the first non-translation ROM hack I've ever played. It was a great SMW hack in its time and can still stand out today but holy shit, Subconian Spy is a pain in the arsenic.