When Donna and the Doctor travelled to the Ood Sphere in 4126 and discovered the Ood's servitude was caused by the removal of the Ood's forebrain and the "third element", the controlling Ood Brain, being blocked. They together unblocked the Ood Brain, ending the chaos between the Ood and humans running the Sphere. The suffering that the Ood went through combined with witnessing Klineman Halpen get turned into an Ood was too much for Donna, who said she wanted to go home; she later calmed down and took this statement back, however. When they parted, Ood Sigma referred to Donna as "DoctorDonna". Neither the Doctor nor Donna realised the significance of the statement at the time. (TV: Planet of the Ood)
Much later, Donna and the Tenth Doctor rushed to Earth in 2009, only to find it taken from underneath them. They then went to the Shadow Proclamation for help. They traced the Earth, along with twenty-six other missing planets, to the Medusa Cascade, where the planets had been hidden by being placed one second out of sync with the rest of the universe. As the Doctor reunited with Rose Tyler, his former companion, he was shot by a Dalek and rushed into the TARDIS with Rose, Jack Harkness, another companion, and Donna. As a result, the Doctor began regenerating but was able to use some of the regeneration energy to heal himself before funnelling the remaining energy into a matching bio-receptacle — his spare hand — and stop the regeneration. (TV: The Stolen Earth)
The TARDIS was taken on board the Crucible and its defences were taken down. Donna was separated from the Doctor when the TARDIS refused to let her leave when the others surrendered to the Daleks. Donna reached out toward the Doctor's hand when the TARDIS was being sent to the core of the Crucible to be destroyed. The regeneration energy stored in the Doctor's hand interacted with Donna and created a being who looked like the Doctor but was half human and had aspects of Donna's personality.
When Donna tried to use the Meta-Crisis Tenth Doctor's Z-Neutrino Biological Inversion Catalyser on the Daleks, Davros electrocuted her. The electrical shock on Donna stimulated her brain, awakened the Time Lord DNA that transferred to her during the two-way biological meta-crisis, and gave her "the best part of the Doctor — his mind," rendering her the "DoctorDonna" of the Ood's prophecy.
She used her new-found intellect to deactivate the reality bomb and disable the circuitry on the Daleks' controls and helped use the magnetron to bring twenty-six of the planets back to their rightful places in the universe. The Meta-Crisis Doctor, fulfilling one of Dalek Caan's prophecies, destroyed the Daleks. When the Supreme Dalek wrecked the magnetron, Earth was the last planet still in the Cascade. Donna helped the Doctor and his companions to "tow" the Earth to its proper place in the galaxy with the TARDIS.
After assisting the Doctor in exiling the Meta-Crisis Doctor to Pete's World and saying farewell to Rose, Donna's mind overloaded, a consequence of the unbearable Time Lord-human meta-crisis. Donna now had a second mind in the form of the Doctor's knowledge and some of his personality aspects, which started to encroach on her original mind and overwhelm it. To save her life, the Doctor was forced to erase her memories against her wishes and bring her home, which suppressed her "DoctorDonna" alter ego. (TV: Journey's End)
On Christmas Day, the Master turned every human into versions of himself. Donna's half-Time Lord mind caused her not to transform. The shock of seeing her mother and Shaun Temple transformed into duplicates of the Master reawakened some of the memories of her travels with the Doctor. Visions of aliens she had met appeared in her mind. She called her grandfather Wilfred Mott and was told to run for her life. She ran, but was cornered by Master duplicates in an alley, and was beginning to break down over the rush of recovered memories. However, the Doctor had also implanted a form of mental self-defence in her mind that released a blast of energy, putting Donna to sleep and knocking the Master facsimiles unconscious. (TV: The End of Time)