By approaching enemies undetected, the player character is able to execute a "one-hit" kill. To facilitate this, the player can extinguish light sources, hide behind objects and pass through narrow grates and shafts inaccessible to enemies. If detected, they must evade their enemies and remain out of sight for a set period of time, after which enemies will return to their original patrol routes and one-hit kills on them will once more be possible. Although fighting hand-to-hand with the goal of quickly disabling an enemy is possible, doing so risks being killed or disabled in turn, as well as alerting nearby enemies with a noisy scuffle. Enemies respond to their environment and one another; they will sound an alarm if they unambiguously spot the player character or find a body, which will alert all enemies in the area and prevent them from returning to their previous patrol routes. If they hear a sound or get a glimpse of the player character they become suspicious, displaying a question mark over their head and actively searching for the player character until they are convinced he is not actually present. They are also subject to terror tactics, and the player can learn to kill and display enemies in a terrifying manner, such as leaving a strangled enemy dangling from a perch or throwing a dead body at an unsuspecting live enemy, causing them to display an exclamation mark over their head, scream, stumble, and fire wildly at anything which scares or startles them.
The player receives points for executing a variety of tactics such as silent kills, passing by enemies without being detected, successfully preventing the discovery of bodies, terrorizing opponents to cause them to fall into disarray, and achieving optional goals such as recovering artifacts and a series of haiku which tell the history of the ninja clan. This score is reduced when an alert is sounded by enemy guards. Depending on the player's performance on the stages, they earn honor and complete seals that are used to buy or upgrade skills and weapons and also unlock alternate costumes. Players get honor by reaching a specific score on each stage, collecting the secret scrolls, and completing the special objectives present in each stage. Some of the scrolls are obtained through secret challenges, where the player is transported to a room to solve a puzzle.
Before each mission the player is taken to an upgrade screen, where they can spend their honor in three sections: Techniques (passive upgrades and skills such as new one-hit executions), Distraction Items (such as smoke bombs and firecrackers) and Attack Items (such as caltrops and poison-tipped darts). The next screen allows the player to select any unlocked costumes and equip items. Each costume (except the starting one) gives special bonuses, but also a side effect in order to balance its power, and must be unlocked by completing a number of seals in the form of optional goals related to the specialty of that costume (for instance, optionally terrorizing enemies in specific ways to unlock a costume which improves the ability to terrorize). Completing the game once unlocks a New Game Plus mode where the enemies behave more intelligently and can kill the player with only a single bullet or hand-to-hand strike, the player's line of sight is limited to a cone aimed in whatever direction the player is facing (preventing players from seeing what is behind, above, or below them unless they turn to look), the player is unable to see the enemy's line of sight and must intuit it from the direction an enemy's face is pointed, and the player can no longer detect the radius at which their actions will be heard by enemies.
Plot
The game's story begins with an attack by a heavily armed force on the dojo of the Hisomu ninja clan. The unnamed ninja protagonist, resting after receiving an extensive irezumi tattoo, is awakened by a female ninja named Ora. Gathering his equipment, the protagonist is able to defeat the attackers and rescue his sensei, Azai, as well as several other members of the clan. Before being rescued by the protagonist, Azai was being blamed by the attackers, who said "you picked the wrong people to steal from". However, Azai simply claimed that the clan has been hidden from the modern world for centuries until now, when "their enemies finally found them". Azai tells him about the power of his tattoo and the legend surrounding it, explaining that the ink comes from a special desert flower and grants greatly sharpened senses and reflexes when absorbed slowly into the body, but will ultimately cause one so tattooed with it to descend into madness. In the distant past, the first Hisomu ninja to receive the Mark became incredibly dangerous, but increasingly deranged, leading to a lengthy reign of terror that only ended with his demise. Therefore, to receive the Mark, a ninja swears to commit seppuku once the madness begins to take hold.
