Through The Portal is an unlockable side mission in Tenchu: Wrath of Heaven.
After Battling with Onikage, Rikimaru sense that Lord Mei-Oh's castle was begining to destroy itself, and that the only way out was through a portal Lord Mei-Oh opened previously. Rikimaru jumped into the portal and was transported to a different realm. This realm was a modern day city. In this realm, people were dying because of a virus, and an evil pharmaceutical corporation was selling an antidote but at a very high price. Many people could not afford this antidote, and died. Rikimaru, thinking about all the people that died because of that corporation decides to take this situation with his own hands, and goes to the main building of the Pharmaceutical Corporation to battle Mr.D.CEO, the head of the Evil corporation.
Rikimaru is the only playable character in this mode. The level is set on a modern day city. The Enemies are not Samurais, Ronins, Demons or any of the enemies of the other levels. The enemies in Through The Portal are Police Officers armed with Night Sticks, which behaves pretty much like the regular Samurais found in the first levels of the main game. Other Enemies found in Through The Portal are Police Officers armed with Pistols, which represent the Bowmen Samurais from the main game. The Police Officers with Pistols are pretty hard to attack, because they carry automatic weapons, the shots take 9 or 8 HP from your life bar, and when assisted by the Night Stick Police Officers it's a nightmare. The Final boss of this mode is Mr.D.CEO who is a cyborg executive that runs the Pharmaceutical Corporation. He attacks with a Ninjato and uses is arm to throw energy balls at Rikimaru. He can also grab Rikimaru and do a pretty powerful move that consists on 4 quick stabs that take 7 HP of the life bar, followed by a haymaker punch and finally a strong slap. This combination takes 25 HP of your life bar. This mode starts you with 5 Caltrops, 2 Shurikens, 1 Medicinal Drink, 2 Poisoned Buns and 1 Bomb.