Mar-Kom was sited in St Paul's Cathedral. It had frozen the inhabitants of New London in time stasis and outlawed time travel as a crime punishable by death. Offenders were executed by being forced to battle a killing machine. In its arrogance, Mar-Kom allowed the condemned to observe the power of the machine and even granted access to a laboratory to prepare weapons to fight it — which did not help them survive. Mar-Kom observed the combats through the eyes of a giant golden android.
The last two time travellers Mar-Kom's chief of police Andron captured were Theophilus Tolliver and the Third Doctor. Working together, they destroyed the killing machine and Mar-Kom's android body. Fleeing and making their way to the Cathedral, the Doctor openly challenged the computer's power. Unused to being defied after so long in power, the shock to the system caused Mar-Kom to self-destruct. (COMIC: The Eternal Present)