In space beyond Earth's atmosphere, a classic battle is waged: Baltan vs. Ultraman. In this case, though, it's Ultraman Cosmos, an Ultra-warrior not directly connected to the other Ultras, who tries to stop the Baltan from reaching Earth. They clash, resulting in serious injury to both. The Baltan is pacified by an SKC team, but another branch tries to destroy it, merely waking it up.
Meanwhile, a young boy named Musashi arrives at his "fort" with friends. He is ridiculed because he believes the fictional Ultraman heroes are real. Yet, after he is left alone, he feels a pull towards a ridge. There he finds the transparent form of Ultraman Cosmos. Realizing what Cosmos needs, Musashi rigs up various reflectors to shine the light of the rising sun on a prominent spot on Cosmos' forehead. This recharges the giant, who repays Musashi by flying him around. Before he leaves, he gives Musashi a stone; by gestures he indicates that Musashi can use it to call Cosmos should the need arise.
Musashi is eventually drawn into a great mystery, involving a mysterious planetoid that turns out to be the Baltan homeworld. (Obviously these Baltans are slightly different from the ones the other Ultra-warriors have fought over the years.) One of Musashi's friends is taken over by a young Baltan, who wishes the Earth no harm. The hotter heads prevail, and the giant Baltan returns to wreak havoc. Using the stone, Musashi calls on Cosmos, who eventually defeats the Baltan after both change forms. The corpse of the giant Baltan is carried back to the planetoid, and the planetoid leaves.
Years later, an adult Musashi joins a branch of SKC known as Team EYES (Elite Young Expert Squadron). EYES' mission is to protect, or "save," monsters whenever possible. Musashi had previously been part of a project on Kapuya Island, where several monsters were already protected; it was from there that he chose to apply for membership with EYES. One of his first missions brings him and the team into contact with the Chaos Header, a virus-like organism that infects existing monsters, often taking control of them and mutating them into deadlier forms; it can also create clones of monsters -- or even Ultraman himself! -- from its own raw material. Musashi discovers that his Ultra-friend has returned to Earth to do battle with the Chaos Header and its creatures; in order to remain on Earth, Cosmos merges with Musashi.
Cosmos starts out with two distinct forms. The first is Luna mode, shown at the top of this article, which resembles a classic Ultra, except the body is mostly blue. Luna mode is considered a "peaceful" mode; because Cosmos would rather settle battles peacefully, this is his main form. Supposedly Luna's abilities are defensive or soothing, although he seems to do quite a bit of physical fighting in this form as well.
The second is Corona mode, shown here. Corona mode appears to be a completely different Ultra, and, in fact, Cosmos Corona has a slightly different focus than Luna mode. Cosmos takes on Corona mode when a peaceful settlement won't work, and he is forced to battle and destroy the monster. Corona mode is more powerful than Luna mode, which makes sense since Corona mode is for destroying monsters.
A direct attack by the Chaos Header drains Cosmos of his energy, leaving him transparent, as he was when young Musashi first found him. Realizing that Cosmos needs courage, Musashi chooses to merge with Cosmos again (after being released by Cosmos for his own safety). This occurs during an eclipse, during which time Musashi and his friends cannot supply enough light to resuscitate Cosmos. A new, more "courageous," form results from this incident; this form is known as Eclipse mode. Eclipse is stronger still than Corona mode, plus wields a beam that can eradicate the Chaos Header virus from a monster, or destroy a Chaos-created clone.
Eventually, Cosmos manages to eliminate the threat of the Chaos Header, once Musashi and Team EYES realize it's nothing more than a pawn of its creator. Cosmos leaves to recuperate, as all the battles against the monsters and Chaos Header have damaged his body.
After an unspecified period of time, a new threat emerges, and Musashi begins to dream of Cosmos. Two new forms of Ultraman Cosmos appear. One is Space Corona mode, a purplish "blend" of Luna and Corona modes. This form seems to be well-suited for battle in space, as, in Musashi's vision, he battles a monster on a planetoid.
Another dream-vision shows a transparent Cosmos Corona apparently battling back on Earth. This mode becomes known as Phantom Cosmos. (Some references call this mode "Skeleton Corona.") The details of these two manifestations are unknown, as they only appear in Musashi's dreams. Eventually, a new threat causes Musashi to call Cosmos back to Earth. The new monster almost beats Cosmos, but he is saved by the sudden appearance of another "legendary giant," Ultraman Justice. The two join forces and defeat the monster.
Later still, a universal intelligence decrees that Earth is to be destroyed. It sends Ultraman Justice to execute its judgement on the Earth. Cosmos is left with no choice but to fight his former comrade, who soundly defeats Cosmos. Musashi witnesses the defeat, and chooses to bond with Cosmos once more, lending him his strength; in so doing, a new Future form results. Musashi and his EYES teammates also discover Justice's human form, a young woman named Julie (or "Jury," since the "l" and "r" sounds are formed the same way in Japanese). Musashi manages to convince Julie that humanity is not the horrid evil it is believed to be. This brings Cosmos and Justice against the latter's former "boss," resulting in the apparent destruction of both. However, it is revealed that their true form is a merged being of tremendous power, Ultraman Legend. Legend is powerful enough to deflect the energy beam set to destroy the Earth. He then splits into Justice and Cosmos once again, and convinces the intelligence that humans are compassionate enough to justify their continued existence as they journey into space. Indeed, Musashi leads the colonization effort by transporting 4 monsters from Kapuya Island; as he leaves Earth, he sees Cosmos, in Luna mode, travelling alongside.