Sentouin, Haken shimasu!

Synopsis

While Tiger Man and Grace's Army defend against the coming invasion, 6's unit drives back into Toris territory to find the secret weapon hidden in ancient ruins. As night falls, they stumble across a camp set up by Heine and Russell. 6 decides to wait until the demons enter the ruins and fight through the traps themselves, then steal the weapon after they do all the work. Upon entering the ruins, 6 and Alice note it looks surprisingly high-tech, while Rose finds some familiar scenery. As Russell and Heine approach the weapon, 6 knocks out Russell from behind with a low blow, but uses too much force, nearly killing him. After Alice revives Russell, 6 makes him activate the weapon, which turns out to be a giant mech. However, Russell activates the mech and puts himself in the pilot seat. Heine teleports away from the scene so she can't be used as a hostage, though 6 grabs her bra at the last second. 6's unit tries to escape outside, but Russell chases them down in the mech.
(Source: Wikipedia)