If you'd wonder what a ninja does when out of his mask, then the film would have painted a rather mundane picture of the practicing ninjas, who hone their skills in enclaves, and frankly look no more like your usual Samurais, except sans status, and honing their skills ala rebels in hiding at their fortified base, and being skilled assassins for hire to Samurais who need someone to do their dirty work for them (yes, even back then you can outsource your dirty laundry), as they themselves are bounded by the honor code of bushido.
The movie doesn't waste time in building up a proper background, but throws you thick into the action. With characters loosely adapted from history, you have Oda Nobunaga, a ruthless warlord on rampaging victories across Japan in an effort to unify the country. Needless to say such aggression doesn't sit well with the able bodied, and 2 clans of ninjas are pitted against each other to see who can carry out a successful mission to stop the warlord. The story centers upon Goemon (Raizo Ichikawa), an up and coming, though ambitious and impatient ninja, who is recognized and granted a promotion (to the back office, away from the battlefield, as an accountant!) but in a moment of lustful folly, becomes the pawn of his master Sandayu (Yunosuke Ito), pledging his life to his master's bidding.
So begins Goemon's mission, which includes a ruining of his reputation, and committing acts which defy even the ninja's code of conduct (yes there is one!). In fact, we learn and observe many rules and regulations of ninja-dom, what with the need to disfigure oneself prior to death, and how torture must be endured and death always an option. All these get interpreted through Goemon's ultimate shame in living with his guilt, up until he meets a prostitute called Maki (Shiho Fujimura), who gives him new cause to live, setting the stage for the truth of his double-headed master to be revealed.
Shinobi No Mono was credited as the first film to popularize the ninja series of films, and had relatively low key special effects, decided to root itself in more realistic elements, rather than have things like tunnelling through sand dunes, and blink and you miss puffing of smoke. Secret passages, booby traps, poison and darts still remain staple, but don't expect any fancy swordplay as targets get dispatched rather quickly. If you're looking for a climatic ending, then you'll likely be sorely disappointed, as everything goes into a big shebang, lacking in any mano-a-mano opportunities.
That said, this film is still rather enjoyable for its shedding of light on these mysterious group who operate in the shadows, and it's not always they have to dress up in black for their operations. It'll look rather dated, but somewhat a refreshing change from current films in its presentation sans the easy way out using tons of computer aided imagery.