How many similarities between The Keys of Marinus and The Key to Time can you think of? 
Here are several for a kickoff:
In the first episode an elder chap (Arbitan/White Guardian) sends the Doctor on a mission to recover key-thingies hidden in distant places which when joined together will help bring about order and harmony... then the chap isn't seen again in the story.
Our heroes are given a device to direct them to the right places (travel dials/Romana's thingyfinder).
A chaotic force (the Voord/Black Guardian) opposes the elder chap and tries without success to impersonate him in the last episode.
A story takes place in an icy environment (Snows of Terror/Ribos).
We see a land of happiness and prosperity which is not what it seems (Morphoton/Calufrax).
In that land there are telepaths (Morpho brain-snails/Mentiads).
A justice machine can serve as an electronic judge and jury (Conscience of Marinus/Megara).
Someone (Sabetha/Cessair of Diplos) is found to be wearing one of the key-thingies around her neck.
There is some connection or other to robot doubles (Edmund Warwick/Androids of Tara).
The Doctor acts as a barrister in a big trial scene (Millennius/the prison ship).
A death sentence is pronounced and then avoided (Millennius/prison ship and Delta Magna).
The big thingy (Conscience of Marinus/Key to Time) that all the key-thingies fit into is destroyed in the end to keep the chaotic force from controlling it.
What else? There must be more.

Here are several for a kickoff:
In the first episode an elder chap (Arbitan/White Guardian) sends the Doctor on a mission to recover key-thingies hidden in distant places which when joined together will help bring about order and harmony... then the chap isn't seen again in the story.
Our heroes are given a device to direct them to the right places (travel dials/Romana's thingyfinder).
A chaotic force (the Voord/Black Guardian) opposes the elder chap and tries without success to impersonate him in the last episode.
A story takes place in an icy environment (Snows of Terror/Ribos).
We see a land of happiness and prosperity which is not what it seems (Morphoton/Calufrax).
In that land there are telepaths (Morpho brain-snails/Mentiads).
A justice machine can serve as an electronic judge and jury (Conscience of Marinus/Megara).
Someone (Sabetha/Cessair of Diplos) is found to be wearing one of the key-thingies around her neck.
There is some connection or other to robot doubles (Edmund Warwick/Androids of Tara).

The Doctor acts as a barrister in a big trial scene (Millennius/the prison ship).
A death sentence is pronounced and then avoided (Millennius/prison ship and Delta Magna).
The big thingy (Conscience of Marinus/Key to Time) that all the key-thingies fit into is destroyed in the end to keep the chaotic force from controlling it.
What else? There must be more.




Have you not seen The Daleks or The Dalek Invasion of Earth?
