Alpha Centauri first came to Peladon on the Committee of Assessment to review their petition to enter the Galactic Federation. It aided the Third Doctor in defeating Hepesh and Arcturus. (TV: The Curse of Peladon)
Five years after Peladon joined the Federation, when Princess Lixgaar of New Mars was taken to Peladon for safety, Centauri locked the Princess away in the tower containing the chamber of the Prisoner of Peladon. It didn't wish to keep this a secret from King Peladon, but was insisted upon by the Federation council to "conceal the truth".
While hiding in the chamber with the Princess, the Third Doctor, who was with King Peladon, Lord Axlaar and Axlaar's accomplice, Sslurn, knocked at the double doors. Centauri opened the doors. The Doctor called for Princess Lixgaar within the chamber, whom Axlaar threatened with his sonic disruptor. The Doctor attacked Axlaar in the chest; the blast of the disruptor hitting Sslurn. Axlaar took aim again at the princess, swatting the Doctor aside, and King Peladon threw himself at him in a blind rage. After this, Axlaar fell from the window.
The next day, Centauri contacted Io. Izlyr arranged a Federation peacekeeping force to take the princess to safety. Centauri apologised for its betrayal to Peladon and announced its immediate resignation. Peladon suggested to Centauri that the refugees would need someone to grant them safe passage home and should relations between Peladon and the Federation continue, "we will need a full ambassador". Meanwhile, the Doctor quietly left in the TARDIS. (AUDIO: The Prisoner of Peladon)
When it became ambassador, Centauri again helped the Third Doctor, this time to defeat the plans of Eckersley and a group of renegade Ice Warriors led by the Ice Lord Azaxyr. (TV: The Monster of Peladon)
It was still serving as the Federation ambassador to Peladon into the 41st century, when it was nearing retirement age. At this time, it escorted Princess Pandora of Earth to be wed to King Pelleas, unaware that she was being led to her death at the hands of Sekhmet. It non-violently stopped Arktos from taking trisilicate from a sabotaged mining operation and helped the Fifth Doctor and Peri return to the TARDIS in orbit around Peladon as a small detour.
Following the death of Pandora, as well as the Martian royalty, Prince Zixlyr and Princess Alixlyr, Alpha Centauri knew it had months of negotiation with the Earth and Martian governments to consider before retiring. (AUDIO: The Bride of Peladon)
Alpha had a squeaky falsetto voice. It was a hermaphrodite hexapod according to the Doctor. (TV: The Curse of Peladon) Nevertheless, the Doctor sometimes referred to it with masculine pronouns, and had a penchant for addressing it as "my dear fellow". (TV: The Monster of Peladon)
Like most of its race, Alpha Centauri was a natural pacifist and a hypochondriac. Despite this, it demonstrated and developed a degree of courage during its time on Peladon, including its success in capturing the known criminal Arktos when he tried to steal Peladon's mineral wealth during Sekhmet's escape. (TV: The Curse of Peladon, The Monster of Peladon)