The Doctor & Co arrive on Marinus and are blackmailed by Arbitan to go on a suicide mission to retrieve the five titular micro-circuit keys.
The Doctor & Co go on a road trip to Peking with Marco Polo. Meanwhile, a man by the name of Tegana is plotting to steal the TARDIS.
The TARDIS suddenly stops and The Doctor, Susan, Ian and Barbara must face their fears and figure out what’s happened.
The Doctor, Susan, Barbara and Ian arrive on the planet Skaro, where war has been raging between the Thal and the Daleks for 500 years.
Teachers Ian and Barbara follow Wunderkind pupil Susan to a junkyard, where they meet her grandfather, The Doctor.