I used to dread the end of rehearsal because then real life would impinge on me. In an effort to find the hidden world, he calls upon a Time Lord-masquerading on Earth as Professor Chronotis at St. Cedd's College in Cambridge-who, in turn, call on his old friend and protege, the Doctor.
Mr Baker played the time lord for seven consecutive seasons, making him the longest-serving Doctor.
The episode was partially filmed in 1979, but an engineering strike prevented the episode from being finished.
Asked why he had returned to the role of Doctor Who for the cameo, Baker said: "Because it never left me, and that's why I can't stay away from it". However, due to strike action at the BBC, production was interrupted halfway through and never resumed.
The newly added scene involving Mr Baker was shot on the set of the 1979 Tardis and lines recorded for the animation are from the original script.
However, it will include footage of Baker as he is today, filmed using 1970s cameras.
The story sees the Doctor with Romana, his companion played by Lalla Ward, working with a retired Time Lord to defeat Skagra, an inmate of the prison planet Shada, who seeks to discover secret information about the prison world. On the 23rd, the BBC tweeted a video of Tom Baker wishing Doctor Who a happy birthday, where he called himself the "happiest" of Doctors.
"Shada" is now available to buy as a digital download and will be released on DVD and Blu-ray on 4 December.
It's exciting to see BBC give makeovers to lost or unfinished episodes of Doctor Who, which still unfortunately boasts huge gaps in the runs of the Second Doctor (Patrick Troughton).
"I loved doing Doctor Who because it was life to me", he continued.