EPISODE: The Mind of Evil: Episode Four
OVERALL EPISODE NUMBER: 286
STORY NUMBER: 056
TRANSMITTED: 20 February 1971
WRITER: Don Houghton
DIRECTOR: Timothy Combe
SCRIPT EDITOR: Terrance Dicks
PRODUCER: Barry Letts
FORMAT: VHS: Doctor Who - The Mind of Evil [VHS] 
Episode Format: 16mm b&w film recording
The Doctor is quickly overcome by the the Keller machine falling unconscious and causing the Master to restart his stopped heart. He is flung into a cell with Jo. The machine then attacks the Master but he locks it in the treatment room and barricades the door hoping to starve it of the negative emotions it feeds on. The Master has the prisoners hijack Unit's missile convoy seizing the Thunderbolt missile. Captain Yates pursues the missile on a motorcycle to a local hanger but is captured. The machine learns how to teleport itself and goes on a feeding frenzy round the prison and attacks Jo & the Doctor........
This episode is built around a location action sequence of the prisoners ambushing the missile which works very well. It's not quite clear how the machine, or rather the creature inside it learns to teleport. If it could do it all along why hasn't it done so before now to escape from the Master? Or perhaps it's got something to do with the amount of negative emotions it's absorbed recently having boosted it's power levels somehow? Either way the wibbly wobbly teleport affect used will be instantly recognisable to anyone who's seen people leave the Liberator's teleport bay in Blake's 7.
Since the very first Doctor Who story the Doctor has spent a fair proportion of his time being flung into cells. In fact the second episode of the first story, the Cave of Skulls, is named after the very first place his imprisoned in. But actual Prisons? You don't see that many of them in Doctor Who. All his companions get locked up in the Conciergerie Prison in Paris but the Doctor stays free. We get an entire Prison Planet in the Dalek Masterplan episode 3, Devil's Planet, an idea that Terry Nation liked so much he reused it in the third episode of Blake's 7, Cygnus Alpha. A sixth season Troughton story, Dick Sharples' the Prison in Space, would have been set in a prison run by leather clad women, but the story was abandoned at a late stage. However later that same season Jamie gets thrown in a Military Prison during the War Games. The only proper prison in the entire series is in this story, but we'll see a prison with a single prisoner in the Sea Devils and one for malcontents on the moon in Frontier in Space. A Prison space ship gets seen in Stones of Blood and another prison in space would have appeared in another cancelled story, Shada.
As you can see we don't return to the subject much after Jon Pertwee's time as the Doctor..... and we'll look at a possible reason for the tomorrow.