The Doctor finally manages to deliver Tegan to Heathrow Airport, where he gets drawn into investigating the in-flight disappearance of a Concorde. Following the same flight path in another Concorde, with the TARDIS stowed in the hold, he discovers that it has been transported back millions of years into the past through a time corridor.

On prehistoric Earth he and his friends encounter a genie-like figure named Kalid and his protoplasmic servants, the Plasmatons. Kalid turns out to be just the latest guise adopted by the Master, who is engaged in a complex scheme to try to gain control of a powerful alien gestalt called the Xeraphin and use it as a dynamorphic generator in his TARDIS.

The Master's interference has caused the Xeraphin to become divided. Its evil side now intends to take over the universe, but the Doctor, Tegan and Nyssa are able to overcome it by combining their will power. The Master, tricked by the Doctor, ends up trapped on the Xeraphin's home planet Xeriphas. The Doctor and Nyssa make a quick departure in the TARDIS, abandoning Tegan at Heathrow.