Terra spends his days as a Keyblade apprentice training under Master Eraqus at the Land of Departure along with his friend, Aqua. Terra's long-time dream was to be a Keyblade master like his master.
Four years ago, Master Xehanort visited the Land of Departure, and brought with him Ventus, a young boy that Xehanort asked Eraqus to take care of and train to be a Keyblade warrior. Terra greeted Ven and asked who he was and why he wanted to be a master, however, Terra was terrified to see Ven cry in raging tears, then fall to the floor. Terra felt a great amount of guilt, believing that it was his fault that Ven became comatose.
After a few days, Ven woke up from the coma. Wanting to make it up to Ven, Terra befriended him and became his older adoptive brother. After a long day of training, Terra told Ven and Aqua about his dream of being a Keyblade master, which was also Aqua's dream. Terra smiled, and gave Ven his wooden sword, saying that he, Ven, and Aqua, all carry the same dream. Terra had been Ven's "older brother" ever since.
The three spend time together whenever possible, realizing each other's dreams and even distributing good luck charms that would always keep them connected.
5 Years later, he takes the Mark of Mastery exam alongside Aqua in order to become a master, but is unable to become one, as he is unable to control his darkness. Unbeknown to them, Master Xehanort is responsible for the appearance of such darkness during the Exam.
Terra was even more surprised to find out that he held darkness in his heart, which cost him a chance to be a Keyblade master. Later that day, he was approached by Master Xehanort, who told him that despite the darkness being in his heart, it didn't mean that it would be a problem. Instead Master Xehanort tells him that darkness cannot be destroyed as Master Eraqus said, but it can only be channeled.
Minutes later, Terra is sent on the quest to find a missing Master Xehanort and stop the threat of mysterious creatures called the Unversed with a promise that Master Eraqus may reconsider him as a Keyblade Master. Aqua is later consulted and told to watch over Terra, as he may be tempted by the darkness.
While on his journey, the darkness in Terra's heart catches the attention of the evil denizens of the worlds he visits. One is Maleficent, the evil witch taking control over Terra through his darkness and causing him to steal a sleeping Briar Rose's heart. It is revealed Master Xehanort had informed the witch of Kingdom Hearts and the requirements to open the final door; Terra bluntly refuses to join Maleficent in her conquest for the six other hearts of pure light. Before he could try getting revenge, Terra was called away by the sounds of a tremor to slay an Unversed. Though victorious, Terra repents for his actions and promises Aurora that he will return her heart once he is strong enough to block the darkness from controlling him.
In Dwarf Woodlands, the Queen also tries to use Terra to kill Snow White and have her heart brought back as proof. However, Terra refuses and ends up in a fight with the Magic Mirror for his life. Though victorious, Terra received a meger clue to Xehanort's whereabouts from the Mirror as his prize.
Returning to his search, Terra had his hope in becoming Master renewed when he met Cinderella and Fairy Godmother, learning that believing in your dreams may be enough; he ended up having fight off an Unversed that crashed the ball. However, it is then that he reuintes with Aqua and learns that Ven is searching for him.
Seeing that he is getting nowhere on his own, Terra seeks the council of one more wise than himself, the great Master Yen Sid. Upon reaching the sorceror's home, Terra watches as Mickey Mosue leaves to assist in solving the world crisis. Terra asks Yen Sid what he should do and is told to resume looking for Xehanort as he may have a conneciton to the boy controlling the Unversed.
Terra's quest through the worlds eventually brings him to the Radiant Garden, where he witnesses Master Xehanort heading in an unknown direction, Terra assuming there may be some new information for him. As Terra treks through the world, a battle with a massive Unversed leads him to reunite with Aqua and Ventus. Terra soon discovering that Ven had encountered the enigmatic Vanitas, he urges the boy to go home, as urgent duties Terra needs to fulfill will make the situation unsafe.
Aqua thinking these duties are not as Master Eraqus had assigned them, Terra quickly tells her that his goal is still to destroy the darkness, to which Aqua counters by stating the fact that Terra seems to embrace it. Ventus quickly comes to Terra's defense, Terra angered when he discovers Master Eraqus had asked Aqua to spy on him, thus learning of his master's mistrust. Leaving his friends behind despite their pleas and Aqua's attempts to persuade Terra he is wrong about Master Eraqus, Terra soon comes to believe that he can only rely on Master Xehanort.
Terra soon locates Master Xehanort in the underground catacombs below Ansem's castle due to Braig, the elderly Keyblade Master seemingly having been captured by the man. Braig battles Terra in an attempt to obtain the Keyblade, nearly victorious, until Master Xehanort shouts to Terra that in order to be victorious, he'll have to use the Keyblade, itself. Terra challenges his words as the Keyblade's use hosts a potential threat to Master Xehanort, forced to concur when the elderly man makes mention of the shame Terra's defeat will bring Master Eraqus, Aqua, and Ventus.
After the battle, Terra gains the upper hand by using his anger as a weapon, thus losing control of his darkness, scarring Braig's cheek as well as ridding the man of his eye. Braig fleeing as he reels in pain, a freed Master Xehanort soon approaches Terra, who is shocked to find he has been controlled by the darkness yet again. Master Xehanort attempts to have Terra see things in a different light, that he has controlled the darkness rather than vice versa, Terra refusing and stating that now he has no home to return to at the end of his journey.
Master Xehanort then proceeds to offer Terra a position as his pupil, causing him to doubt Master Eraqus. Master Xehanort then explains how darkness is always born within the light, revealing that fear is why Master Eraqus failed to pass Terra in his Mark of Mastery Exam. Master Xehanort then begs Terra to join him in bringing balance to the worlds, stating their mission is to defeat the darkness that upsets the balance, Vanitas. Master Xehanort then pulls the final string, acknowledging Terra's long-time wish and calling him "Master Terra". It is because of this that Terra comes to know Master Xehanort as his new master. As Terra prepares to leave the Radiant Garden, Ventus catches up with him, asking to come along on Terra's journey, to which he refuses. Terra does say, however, that he knows Ventus will be there when he is really needed.
As he continued journeying through the worlds (specifically Olympus Colesium and Disney Town), Terra learns that using darkness need not be the only way to fight and achieve his goals.
In Disney Town, Terra walks in and sees Unversed on a race track. He unknowningly walks in while a race is going on and is nearly run over by Pete. After a brief talk with Queen Minnie, Chip and Dale, Pete (posing as Captain Dark) accuses him of being another racer and challenges him. Chip and Dale beg Terra to race as Pete 'needs to be taken down a notch or two' and has been seen to cheat and break the rules. Terra accepts. When Terra wins the race, Pete vows to be back to 'clobber all of ya'. When Pete leaves, Chip mentions the Million Dreams Award (An award given to the most exemplimentary citizen). In the end, Terra learns that he doesn't have to bend the rules to reach his goals and he doesn't have to jump into the darkness.
In Olympus, Terra saves a young Hercules from a pack of Unversed. Hercules mentions the Games going on at the Coliseum. Terra decides to enter to test his own strength. Upon entering, Hades is planning on giving Zeus 'a jolt of his own medicine', but needs a warrior to help him. He sees Terra and decides he's perfect. They at first talk about the darkness inside. Hades says that Terra needs to face his darkness or it will run him over like a 'debutante at a toga sale'. Hades also charms Terra by saying he has potential to conquer the darkness inside of him and has a few pointers to give. Terra ultimately accepts Hades's offer and enters the Games. Their deal turns sour and Hades decides to get a new pawn. He possesses a young warrior named Zack. Terra defeats Zack the first time. Later, Zack is possessed and starts attacking Terra against his will. Terra defeats Zack again and frees his heart of the darkness. Zack thanks Terra gratefully and notes that Terra is what he pictured a hero would be. The audience also thinks the same and considers Terra their hero. In Deep Space, he ends up being taken into custody by the Galactic Federation for passing by a prisoner transport ship. Freeing himself during and Unversed attack, Terra also frees Dr. Jumba, who offers to have his creation, Experiment 626, help defeat Unversed. However, upon freeing 626, Terra has his good luck charm taken from him; he pleas for it to be returned to him, surprising himself how much he misses his friends. Terra wonders if 626 wants to have friends like everyone else, prompting Jumba to release another of his creations, Experiment 221, to attack him in retaliation. 626 tries asking what a friend is, but Terra tells him that it's something that can't be explained, only experienced. With an alarm announcing the escape of all three of them, Terra flees back into the Lanes Between.
In Neverland, Terra saves Captain Hook and Smee from Unversed; he finds Vanitas is not in there, but another boy after the "light" in Hook's chest. Escorting Hook to Skull Rock, Terra is left to guard the "light" while Hook goes to look for a shooting star. Soon after, Terra meets Peter Pan, the boy after the "light"; midway in the fight,Slightly and Cubby try stealing the chest, but drop it, causing treasure to fall out, showing Terra he was tricked into guarding loot. Peter Pan leaves to save Tinker Bell, who had also gone after the shooting star. Hook arrives with Tinker Bell captive in a lantern. Terra frees her and prepares to fight Hook, but the crocodile scares him away. Terra leaves the cave, but the Lost Boys' screams draw him back in the cave, where a swarm of Unversed are attacking. After defeating the Unversed, Terra encourages the Lost Boys to put things they treasure in the chest to replace the treasure lost during the skirmish.
Some time later, Terra finds himself encountering a warm light which calls him to another world. Discovering Paopu Fruit growing on the island, he recalls his friends and the distribution of the good luck charms at the beginning of his quest, Terra wondering why he is there and if he'll ever see Aqua and Ven again. Soon laying eyes upon a young Sora and Riku, he discovers Riku's potential and holds a conversation with the boy. Terra asks why he wants to go to other worlds, later choosing Riku to be his successor when he is able to hold the Keyblade. Riku stating countless times he has cherished things to guard, Terra then makes note that he still has things to protect, as well.
Called soon after to the Keyblade Graveyard by Master Xehanort, Terra learns from the man that Ventus has learned the truth about his past, has gone back to see Master Eraqus about it, and now has a high chance of being eliminated. Terra's goal to protect Ventus, he leaves without question and returns to the Land of Departure. He battles his master, no longer caring about him, using his darkness to win the battle and save Ventus by throwing the boy into a portal to another world. Master Eraqus then dies in Terra's arms from an ambush made by Master Xehanort, Terra suddenly realizing what he has done and beginning to mourn for his master. He then witnesses his home get destroyed as it falls into darkness courtesy of Master Xehanort.
Eventually, the time for the final battle with Master Xehanort and Vanitas had come. Refusing to join up with Master Xehanort and discovering what has become of Ventus's heart, Terra is soon told as Vanitas goes after Ventus and Aqua that he can't save his friends, Terra's great rage and anger towards Master Xehanort staining his heart in the darkness. Terra battles the elderly man, who makes note of the χ-blade's creation before unlocking his heart and possessing Terra's body, thus forming the entity known as "Terra-Xehanort" (portmanateaux by fans as "Terranort"). Terra-Xehanort walking away from Terra's discarded armor, which had been used by the young man in an attempt to shield himself from Master Xehanort's heart, he soon finds himself trapped by the armor, which has reassembled due to Terra's mind. The animated armor fights and defeats the possessed Terra, kneeling as the man is sent to the Radiant Garden and Kingdom Hearts vanishes above it.
Aqua's quest to find Terra after learning from Yen Sid that his heart has been lost, she soon finds him in the Radiant Garden. Surprised when the man attacks her, she soon discovers Terra to be no more, her old friend now a younger incarnation of their old enemy Master Xehanort. Battling the possessed Terra before her in an attempt to return his heart, the dark and confused being that had once been her dear friend proceeds to summon Master Xehanort's Keyblade and a dark symbiote. During the fight, Terra-Xehanort is stopped when Terra's heart begins to fight back against Master Xehanort's from within his body, Master Xehanort's stronger influences causing Terra-Xehanort to unlock his heart in an attempt to finish Terra off. The Guardian then reappears behind Terra-Xehanort, it and the young Keyblade wielder fall into the Realm of Darkness. Aqua gives chase and sacrifices both her Keyblade and armor to save her old friend's body. Terra-Xehanort is brought back to the Radiant Garden, but this time without any memories besides a name: "Xehanort".
Sometime later, Terra's heart is seen with Master Xehanort's inside the new Xehanort's body, the two disputing over who will control it. Terra, with the heart of Master Eraqus sealed within him, concludes that no matter what, he will do all in his power to hold Master Xehanort at bay, no matter how sure the elderly Keyblade Master was that his evil plans would succeed.