Muldwych was an incarnation of the Doctor who spent over five centuries exiled on Antýkhon, the name given to Earth during a period of its history when it was inhabited by Hairies and the Charrl. He was occasionally visited by the Seventh Doctor.
The Charrl came to Antýkhon to establish a new homeworld, but found that it was too polluted and hot for their long-term survival. They did not have enough energy to leave the planet in their gravity ships, so they were forced to stay. As the years passed, Muldwych helped the Charrl and became a trusted ally. During this time, Muldwych began manipulating events so that he would eventually escape Antýkhon.
Ch'tizz, the queen of the Charrl, made a nine day journey to Muldwych's hut. She proposed that Muldwych help guide the Charrl to create a temporal link between Antýkhon and a more suitable period of Earth's history. Planning on using this link to escape, Muldwych agreed.
For 500 years, Muldwych guided the Charrl chronomancers. He helped them hone into the Great Divide and create a link to Russia in 1908. Several Charrl were sent through the Divide, but were all fatally injured by the journey. After seven months, the Divide moved to London's East End in 1909.
Shortly afterwards, Muldwych was visited by the Seventh Doctor. Muldwych asked his younger self for help, but the Doctor refused due to the first law of time. Knowing Ace's eventual fate, Muldwych gave the Doctor a present for her: Madame Bovary, a 19th century French novel. The Doctor was aware that Muldwych needed a TARDIS to stabilise the Divide, so he split his in two and sent the halves to the two sides of the Divide.
The half carrying Ace landed in Antýkhon and was quickly retrieved by Muldwych after Ace left it. Muldwych needed the TARDIS to be complete to stabilise the Divide and went through it with Ace, Seeba, and Korin. They landed in Hanbury Street and met up with Bernice Summerfield. Muldwych reunited the two parts and the Divide was stabilised, letting all the Charrl come to 1909. With the TARDIS complete, Muldwych told the Charrl that they couldn't take over Earth in 1909. He told the Charrl he would take them to an alternate Earth where humanity never evolved, but he actually took them to an unused part of the Seventh Doctor's TARDIS.
With the Charrl gone, Muldwych could finally reach the goal of his masterplan: to use the Seventh Doctor's TARDIS to escape Earth. However, after Muldwych set the TARDIS to dematerialise, the TARDIS sent Mukdwych back to Antýkhon.
Muldwych managed to escape Antýkhon when Time, Pain, and Death came to Bernice Summerfield and Jason Kane's wedding through Puterspace. Using "a narrow beam of chronons perfectly aligned with the marginal reality established by the Puterspace apparatus", Muldwych came to Cheldon Bonniface on 24 April 2010. Along the way, he found himself In another marginal reality and met and befriended a Unicorn named Bat.
Muldwych had a plump figure and a ruddy face. During his exile, his hair was grey-brown, (PROSE: Birthright) while after his exile, it was mostly grey with streaks of red. He wore a white robe (PROSE: Happy Endings) and had a blue Roman-style ring on his left hand. (PROSE: Birthright)