He is initially sceptical but as he comes under scrutiny he wonders if they have stumbled on a secret powerful people don't want exposed. Meanwhile Mika's investigation leads her to Raizo, a ninja who has left the clan. Now the rest of the clan are determined to kill him for his betrayal and eliminate anybody who might expose them. Inevitably this will lead to much bloodshed before a final confrontation. Interspersed with this we see a succession of flashbacks which show Raizo's upbringing in the clan, its brutal training regimen and the reason he ultimately left them.
If you are looking for subtlety you are in the wrong place; this film is full of bloody ninja action which sees limbs severed, heads cut in two and blood all over the place. This starts in the opening scene so if you can get past that you should be okay with what follows.