is a Japanese anime series animated by AIC and created by Masaki Kajishima, best known for the Tenchi Muyo! franchise. It was originally broadcast on Wowow from April to July 1999 and was licensed in the US by Pioneer LDC, later known as Geneon. In addition to the original TV run, an OVA special was added to conclude the series as a de facto "14th episode".

The title refers to the fact that the story takes place in two parallel universes, a concept loosely based on multiverse theory.

Plot 22 years ago, a huge alien artifact is found on a construction site by a worker named Yotsuga. The foreman orders that the artifact be broken apart to avoid delays. As Yotsuga walks towards the dumpsters to dispose of a small piece of the artifact, the world splits into two. In one world, Yotsuga tosses away the piece and the artifact is destroyed. In the other world, Yotsuga keeps the artifact, leading to the discovery of extraterrestrial technology that changes history.

In the present, Kazuki Yotsuga is a high school student who is shunned by other students as an otaku; his web site describes battles between giant robots that only he can see, but it is mostly as a result of him being occasionally seen by the other students reacting to their "presence". One day, the most popular girl in school, Mitsuki Sanada, approaches him to tell him that she likes his stories. This is only a ruse; she lures him to her father, Dr. Ken Sanada, who wishes to study Kazuki for his research on parallel worlds.

After being thrown into the other possible world by Dr. Sanada's device (thanks to a careless action by Mitsuki), Kazuki finds himself still in Dr. Sanada's basement, but everything is covered with cobwebs. Kazuki goes home, is distracted by a real battle between two giant robots, one white and the other black. The white robot is knocked down and an injured female figure emerges from inside. When Kazuki goes to her aid, he is trapped inside the robot and somehow manages to activate it and defeat the enemy robot. Afterwards, Kazuki reaches his home, but his parents no longer recognize him. He is found by Mitsuki, who has followed him. Along with a military escort, she takes him to the Earth Defense Force HQ, a paramilitary operation led by this world's Dr. Sanada. After Kazuki learns of this world's situation and the attempted conquest of this world by this world's Dr. Rara (Dr. Sanada's arch-rival) who is using technology developed from the artifact, he volunteers to become a giant robot pilot.

Over the course of the series, Kazuki manages to attract several very different women without even trying, and he becomes a hero. He is later taken prisoner by Dr. Rara, the father of this world's Mitsuki. Mitsuki Rara eventually defects and comes to live with Kazuki and her counterpart. Later on, a new type of robot made from the artifact is deployed that can cause objects and people to be shifted between the alternate worlds. During the battle against it, Kazuki and Mitsuki Sanada are sent back to their original world, and they discover their world's Dr. Sanada covered up their disappearance by claiming that they had eloped. As it turns out, the frequent travel between worlds is taking a toll on the universe itself as the barriers separating them begin to break down; this will eventually lead to both worlds vanishing. A decision must be made; since Kazuki and the alien technology are exclusive to each world, one must be eliminated to halt the destruction. The robots are destroyed, and the worlds combine into one, an expression of Kazuki's wish for all his (girl)friends to continue to stay together with him (even though the action is again caused by Mitsuki's carelessness).

Masaki Kajishima, confirmed that DUAL! is in fact set in the distant past of the Tenchi Muyo! universe.

Characters

Kazuki is the main character of the series: a shy, quiet and awkward everyman, who possesses incredible fighting power and illogical charisma. As such, he attracts a bevy of beautiful women. Kazuki is regarded as a pariah by his classmates because he periodically has visions of giant robots battling in the city. After being accidentally sent to the parallel world by Ken and Mitsuki Sanada, Kazuki discovers that he is the only male that can control the mechas. Because of his unbelievable piloting skill, he is commissioned to the Earth Defense Force, or simply the EDF, and becomes the new pilot of the white Core Robot. His cover story at school is that he is Mitsuki's little brother, which gives Kazuki quite a bit of trouble.

Curiously, there is no alternative version of Kazuki Yotsuga in this world. Later in the series, Kazuki subconsciously awakens one of the original alien mechas, a robot named Zinv. It is revealed that Zinv is Kazuki's counterpart in the parallel world.

When Zinv is destroyed, Kazuki's desire for the group to stay together results in the creation of a third possible world, where elements from the parallel universes are combined.

Mitsuki is the most admired girl in Kazuki's school; young men constantly send her gifts and cards, and Kazuki is nearly lynched for speaking to her. She lures Kazuki to her house for her father's experiments, and later follows him into the parallel world (for some reason arriving a month before he does). She is the daughter of both Ken Sanada and Ayuko Sanada in this world; however, she is in no way related to the Ken Sanada from the parallel world.

Mitsuki is also an exceptional robot pilot, and joins the EDF to end the war, hoping research to send her home can begin afterwards. Unfortunately, Mitsuki is hotheaded and easily manipulated, and is captured by the Rara Army towards the end of the series and coerced into piloting the complete version of HIMC.

She clearly has an attraction to Kazuki, but covers it up by treating him abrasively. When Kazuki is lost behind enemy lines, Mitsuki is affected most of all.

D is a bioroid, an artificially constructed biological/mechanical being first introduced in the Tenchi Muyo! universe. The war between the Earth Defense Force and the Rara Army began when D and at least two mechas were discovered in an artifact of unknown origin. Her original body was damaged, so a bioroid was built to contain her essence (represented by a glowing orb in place of an eye) while her body healed.

D appears to be as inhuman as the robot she pilots, speaking mechanically and asking many questions about human emotions and behavior. The only one to break through her stony exterior is Kazuki, to the shock of Commander Sanada and Mitsuki. D frequently has visions of a horrible creature attacking, and is deathly afraid of HIMC.

She develops a strong attachment to Kazuki, culminating in her decision to return to her real body at the climax of the series in order to save him, at which she came to the realization that she was in love with Kazuki, a new feeling she had not experienced or understood before. It is then revealed that D stole Zinv and fled her world to escape HIMC and Kumu, who claimed the lives of her friends.

As a robot pilot, D is subpar, often requiring rescue by Kazuki and Mitsuki. However, she redeems herself by removing the limiter placed upon Zinv, allowing its true power to manifest itself.

When Zinv is destroyed and the worlds merge, D becomes Dee Sanada, taking on the animated personality of a young girl.

There is also an apparent resemblance between D and Tenchi Muyo! GXP character Neju na Melmas, leading some to believe that she and D may belong to the same race. The character of D also resembles the character Doll from Tenchi Muyo! War on Geminar. D will appear in the Tenchi Muyo! GXP OVA sequel Tenchi Muyo! GXP Paradise Shidō-hen.

Yayoi is the original pilot of the white Core Robot, which Kazuki named "Hartzenen". After suffering a serious head injury in battle, Yayoi experienced a feedback loop, effectively cutting off her connection to robots and ending her career as a pilot. Kazuki climbed into Hartzenen and took control, saving Yayoi's life. She refers to Kazuki as "her prince", and falls in love with him.

Yayoi goes to great lengths to spend time with Kazuki, signing up to be his combat instructor and school homeroom teacher, and even moving in next door to the Sanada home and building a passageway and door into Kazuki's room (immediately boarded up by Mitsuki). Her grandmother once tried to arrange a marriage for Yayoi to another man (due in part to Mitsuki Sanada's meddling), but her affection for Kazuki was too deep for her to even pay attention to the would-be suitor.

When Zinv is destroyed and the worlds merge, Yayoi is Kazuki's homeroom teacher, still living next door and usually entering the Sanada house through Kazuki's window (as a short cut), to Mitsuki (Sanada)'s annoyance.

The Earth Defense Force knows her as "Miss Ra", the Rara Army's spokeswoman/mascot, but when Kazuki is injured and trapped behind enemy lines, it is revealed that she is Mitsuki Sanada's counterpart in the parallel world, the daughter of Hiroshi Rara and Ayuko Rara.

Mitsuki Rara could very well be Mitsuki Sanada's direct opposite. She is quiet, easily embarrassed and gentle; she personally nurses Kazuki back to health. However, she places herself in a trance to play Miss Ra, becoming a ruthless warrior. When she finds out that Kazuki is an enemy pilot, she launches him away in an escape pod, remembering the time he could have killed Miss Ra, but saved her instead.

Mitsuki is later brainwashed into piloting HIMC, one of the most powerful robots created. After Kazuki saves her, she resists returning home, eventually becoming a Core Robot pilot for the EDF. She comes the closest to capturing Kazuki's heart; the two almost go out on a date, which is unfortunately cancelled by U.N. officers intent on sending her home.

When Zinv is destroyed and the worlds merge, Mitsuki lives with her parents right next door to the Sanada home, but has a habit of sneaking in Kazuki's window via sleepwalking and spending most of her time in the Sanada home.

Ken has A and B versions: the eccentric scientist from our world, and the parallel world's commander of the Earth Defense Force. Ken is Mitsuki's father in this world, but is childless in the other world; naturally, he is quite confused when our world's Mitsuki calls him "Dad".

Both Kens share a love for instant ramen, a slightly off-color sense of humor, and an understanding that Kazuki is special. Despite appearing to goof off on the job (antagonizing Rara and Akane never gets old), Commander Sanada takes it quite seriously. When the U.N. turns against the EDF, his subordinates remain loyal to him, proving his worth as a leader.

When Zinv is destroyed and the worlds merge, Ken Sanada returns to being a scientist as well as being married to Akane Yamano. In the new world, Mitsuki Sanada is his daughter and Dee Sanada is his adopted daughter. His relation to Kazuki is cause for question. Kazuki calls him "Uncle" but it is debated among fans whether they are actually related in the new world.

The Earth Defense Force's United Nations advisor. Despite Akane's tendency to work "by the book", Ken and the young pilots often convince her to defy her orders, such as Rara's demand for the EDF to repatriate his daughter. When the EDF is outlawed, she resigns her post and flees with Ken Sanada and the others.

When Zinv is destroyed and the worlds merge, Akane becomes Ken's wife, as well as (unfortunately) Ayuko Rara's junior (as Ayuko was married to Ken previously), enabling Ayuko to insist on Akane's calling her "big sister".