Synopsis

Mys­te­ri­ous events involv­ing dis­ap­pear­ing oil rigs and a secret Defense Min­istry project attract UNIT’s atten­tion, espe­cial­ly when the Mas­ter’s involve­ment becomes appar­ent. The Mas­ter is being used as a con­sul­tant by a Min­istry of Defense com­mu­ni­ca­tions project but in real­i­ty he is using them to broad­cast a tachy­on res­cue sig­nal to his past and future selves. How­ev­er, the rapa­cious alien race of the Sild inter­cept the sig­nal and use it to pluck var­i­ous incar­na­tions of the Mas­ter out of time, start­ing to erase him from exis­tence. Then the alien inva­sion begins, whose object is to cap­ture the Mas­ter him­self as the Mas­ter Stroke of their Mas­ter Plan of cre­at­ing the Mas­ter Com­put­er, built of all the incar­na­tions of the Mas­ter they were col­lect­ing. ¹ They don’t find him, thanks to the inter­fer­ence of the Doc­tor who came to res­cue him, which only leads to the aliens cap­tur­ing him any­way in the far-flung future. But that is exact­ly what the Mas­ter wants, because the Mas­ter is in con­trol of the com­put­er, not the Sild, as they discover.