Along with the other council members, Trantis was determined to conquer the Solar System with the help of the Daleks. He represented the largest of the Outer Galaxies, making him the most powerful member of the alliance after the Daleks, and he attended the Intergalactic Conference of Andromeda on behalf of all the Outer Galaxies.
To neutralise Trantis' influence, Mavic Chen suggested he was behind the theft of the taranium core, reminding the Council that Trantis' people had experimented unsuccessfully with time travel. The Daleks did not suspect Trantis of involvement but were wary of his arrogant dismissal of the Daleks' efforts to retrieve the core. When the Daleks got back the taranium core from the First Doctor, they tested their Time Destructor on Trantis, with Chen and Celation as witnesses. When the device did not kill him, they discovered that the Doctor had given them a fake core. They then exterminated Trantis as he exited the chamber. (TV: Mission to the Unknown / The Daleks' Master Plan)
Behind the scenes
Representative Trantis' appearance changed during the course of the alliance; publicity photos from Mission to the Unknown show Trantis as a hunched green alien with dark hair and with short tendrils hanging from his face; "Day of Armageddon" (the only surviving episode of The Daleks' Master Plan to feature Trantis) shows Trantis looking much like a normal human, except for his jagged, sharpened teeth. Although Ronald Rich is often credited with playing Trantis in Mission to the Unknown, he is in fact the tall, muscular alien in the silver helmet seen in many publicity photos from that episode. Johnny Clayton plays the smaller character with the tendrils, which is a closer match to the later Trantis played by Roy Evans in "Day of Armageddon".