Doctor Who: Arc of Infinity

Future Doctor Colin Baker makes his Doctor Who debut as Maxil - and gets to shoot the current incumbent of the lead role in the closing scene of Part One.

Traveling with Nyssa in the TARDIS, the Doctor is attacked by a malign entity – a being of pure anti-matter seeking to cross the dimensions. Although the invader is successfully repelled, the TARDIS is immediately recalled to Gallifrey, where the High Council of Time Lords sentence the Doctor to be executed to prevent any further attempts at bonding. It seems there is a traitor on Gallifrey. And what links the Renegade and the disappearance of Tegan's cousin in Amsterdam? Two fates intertwined must battle for the future of the entire universe...

An antimatter creature has crossed into normal space via a phenomenon known as the Arc of Infinity but needs to bond physically with a Time Lord in order to remain stable. A traitor on Gallifrey has chosen the Doctor as the victim.
The High Council, headed by President Borusa, decides that the Doctor's life must be terminated in order to avoid this danger. Tegan meanwhile arrives in Amsterdam to visit her cousin, Colin Frazer, only to learn that he has disappeared. She enters a crypt in search of him and is captured by a hideous creature, the Ergon.
On Gallifrey, the traitor is revealed as Councillor Hedin and his master as the legendary Time Lord figure Omega, who has been trapped for centuries in the universe of antimatter. Omega seizes control of the Matrix. The Doctor however is able to trace him to Amsterdam thanks to clues received via the Matrix from Tegan - who has been taken to Omega's TARDIS by the Ergon.
Omega's body is turning into a replica of the Doctor's, but the bonding is incomplete and there will soon be a massive explosion as he reverts to antimatter. The Doctor, Nyssa and Tegan chase Omega through Amsterdam and eventually corner him.
Omega wills his own destruction, but the Doctor fires the Ergon's matter converter at him and he fades harmlessly away.