1. Obito, dead and trapped under rubble, is found by Zetsu shortly after his death, who then brings news of Obito's whereabouts to Madara.
2. Madara, who realizes that his death is drawing near (possibly due to limitations / side effects of the Rinnegan), has need of a vassal to carry out his plans and ensure his revival.
3. Madara uses his Rinne-tensei, perhaps for the first time, to revive Obito, whom Madara then augments with Hashirama's cells to repair Obito's devastated body.
4. Madara proceeds to brainwash Obito in much the same way that Tobi seeks to brainwash Sasuke. Obito is unstable after having died, and filled with bitterness after learning of Kakashi's failure to honor Obito's sacrifice and protect Rin. Latching on to Madara's vision of the Moon's Eye Plan, and filled with Madara's hatred of the Senju and disdain for the concept of "hope", Obito becomes to Madara what Kabuto was to Orochimaru, a pawn who seeks to both imitate and surpass his master. However, unlike Kabuto, who sought to fill the gap in his identity by stealing Orochimaru's, Obito seeks instead to rid himself of Madara's shortcomings by becoming a "perfect" Madara, No One.
5. During this time of grief, and under the tutelage of Madara, Obito has already unlocked the power of his single Mangekyou, granting him the warping ability. He is tested in much the same way that Tobi himself tested Sasuke, but has a much easier time letting go of his own identity and falling completely into darkness. However, due to the limitations of his body, Obito is not yet ready to wield the power of Madara's Rinnegan, which instead is transferred to Nagato just before Madara's death.
6. Obito is charged with managing the Moon's Eye Plan in Madara's absence, and manipulates Nagato's group to form the Akatsuki.
7. However, Obito is overly anxious to get things under way, and launches the Kyuubi attack against Konoha; the attack is personally symbolic to Obito as a way of proving his superiority to the real Madara, who's own Kyuubi attack failed. Obito also still feels bitterness towards Konoha. However, upon failing to destroy Konoha or keep his hold on the Kyuubi, Obito is forced to wait for Nagato and the Akatsuki to mature, and just like Madara, gives up the idea of acting on his own. Obito adopts the Classic Tobi philosophy of manipulation over personal involvement in battle.
8. In the wake of the Kyuubi attack, Obito manipulates the hatred within Konoha to destroy the Uchiha Clan. Doing so, Obito carries out vengeance against the clan for succumbing to the Senju and betraying Madara, and also grants Obito access to countless Sharingan (the True purpose of which remains unknown). Also, the Uchiha are the only clan capable of posing a threat to the Bijuu that Obito intends to collect, as well as other aspects of Tobi's war.
9. With time, Obito begins to realize that he can use Sasuke, a healthy, vulnerable young Uchiha with the potential to truly surpass Madara, to complete the Moon's Eye Plan on his own. Obito strays from his role as a mere vassal, and instead seizes the opportunity to become the Master instead of the servant. He even develops an obsession with forcing Sasuke to follow Madara's footsteps, which represents Obito's surpassing of Madara by becoming the Master. Sasuke, ruled by the same hatred and pride that ruled Madara (and rendered him imperfect, in Obito's eyes), is easily lead down this path. Sasuke also represents, to Obito/Tobi, the unstable boy that Obito had been, and so Obito purges himself of that memory by acting as "Madara". He even uses the same speeches that Madara once used on him, posing as Madara and usurping Madara's history and identity.
10. Unfortunately for Obito, Kabuto has revived the real Madara, whom Obito still fears; due to his own limitations, Obito cannot wield the full power of Madara's Rinnegan, which he uses only sparingly, and only in one eye, relying instead on the power of his Sharingan. With Sasuke far from ready, and the war already in motion, Tobi is forced into an uneasy alliance, but rushes to accomplish his plans before Madara has a chance to intervene.
11. Along the way, Obito's greatest obstacle is Naruto
, who represents everything that Obito despises. As the heir to the Senju Will of Fire, Naruto represents the ancient enemy of the Elder Brother Ideology. As the son of Minato and the Kyuubi Jinchuuriki, Naruto represents Obito's past failure to prove himself superior to Madara. What is more, Naruto is the son of the sensei Obito betrayed, and Naruto's own sensei is Kakashi, the man Rin truly loved, the man who now holds Obito's missing eye and renders him incomplete, and the man who's broken promise destroyed Obito's last threads of hope/love. Naruto is virtually immune to the story of hopelessness and hatred that manipulated Obito, Nagato, and Sasuke. Nagato, in seemingly no time at all, chose to give his life betting on Naruto's Hope, throwing aside the careful brainwashing that Obito himself had succumbed to. Naruto is to Obito what Hashirama was to Madara, his Achilles Heel. Naruto's very existence threatens to disprove the "No One" philosophy that defines Obito, and also threatens the entire Moon's Eye Plan. Most importantly, Naruto is the last person with the potential to influence Sasuke; while Obito never had a loyal friend to help him fight against Madara's ideology of hatred and hopelessness, Sasuke still has Naruto to remind Sasuke of who he truly is. While at first Obito was determined to use his Mini-Madara, Sasuke, to destroy Naruto and everything Naruto stands for, Obito is now forced once again to risk himself in battle, due to the pressing threat of Madara's intervention and Naruto's own growth in power.