The Master has obtained a TIRU, a device with the ability to chop and splice time, allowing him to rearrange events in the universe. He has holed up in the "Mines of Terror" on Rijar, where Heatonite, a mysterious compound vital to the construction of a TIRU, is mined.
The Time Lords send the Doctor to the mining complex. As an emissary of the Time Lords he may not bring any weapons, but he does have a highly sophisticated robotcat called Splinx.
Searching around the mines, labs and other mazes, the Doctor and Splinx collected various objects to confront and defeat the Master.