A Type 50 TARDIS was the TARDIS that the First Doctor used prior to his life as a renegade Time Lord. (AUDIO: Prisoners of Fate) The Doctor decided to steal another TARDIS, a Type 40, though he nearly stole another capsule when he finally departed Gallifrey with his granddaughter Susan Foreman though he was advised by a version of Clara Oswald to take the Type 40. (TV: The Name of the Doctor)
The Type 50 was angry and hurt it had been spurned by the Doctor when he stole his Type 40, an inferior model, and called it miserable and dilapidated with faulty circuits compared to itself. The Doctor would later claim he had to take the first available TARDIS he could find and he was reluctant to tell anyone why.
The Type 50 TARDIS later escaped Gallifrey on its own, suffering damage in its escape as it attempted to go after the Doctor. The TARDIS had to jettison 9 tenths of its interior to escape, and travelled to Valderon, where the Doctor encountered it during his fifth incarnation in 3556. By that point the Type 50 TARDIS was so badly damaged it couldn't leave the planet, or so it claimed. In fact it was capable of moving short distances. The telepathic circuits had also survived, and with them the TARDIS looked for someone who could tend to it.
The TARDIS was in pain by the time that it encountered the Fifth Doctor again. Sibor had discovered the TARDIS's capabilities of looking into the future after it had granted the people of Valderon with the Miracle, where everyone could speak with a different language. However Sibor decided to use the TARDIS for her own gain, putting it in a prison for 20 years and giving it just enough power to stay alive. According to the TARDIS Sibor tortured it to ensure its obedience. But she didn't realise the TARDIS was showing her possibilities of the future to find the right person to repair it.
During this time the TARDIS claimed it just wanted to return home, but it generated a strong telepathic field to find someone who could repair it. With the telepathic circuits, the TARDIS took control over Tegan and Turlough to communicate with the Doctor.
When Sibor reconnected the Type 50 to the power grid, it escaped and killed her. The Doctor tried to look for it, only to discover the Type 50 TARDIS was still on Valderon, and had assumed the shape of Tegan and then Sibor and took over the Doctor's TARDIS and took them back through time to change Nyssa's history to create a new timeline which would release temporal energy to restore itself to full power. The Type 50 boasted this as it's "emancipation." The Type 50 revealed it didn't want to return home at all, that it was a lie. It stole and "ate" the Type 40's dematerialisation circuit as a temporary replacement for its own circuit, and leaving the Doctor to suffer the consequences of abandoning it 600 years previously.
When speaking to the Doctor about three of his upcoming lives, the TARDIS described the Sixth Doctor as the "the rhetorician," the Seventh Doctor as "the schemer," and the Eighth Doctor as "the idealist."
The TARDIS revealed it had manipulated events so then it could feed off the energies of paradoxes to rebuild itself. The TARDIS was later destroyed when it became trapped in one of those paradoxes. (AUDIO: Prisoners of Fate)