Temmosus was the leader of the Thals on Skaro. They had lived on the plateau for centuries, following the neutronic war which had devastated their world. When the rains which his people depended on for their crops ceased to come, the Thals faced starvation. Desperate to save his people, Temmosus led a party of them to find the city of their ancestral enemies, the Dals. Temmosus hoped the Dals would provide them with food, and perhaps even work with the Thals to rebuild Skaro and restore it to the paradise it once was.
As the Thal party drew near to the Dalek City, the young Thal Alydon scouted ahead. He found the Doctor's TARDIS in the jungle from which a group of four strangers emerged. Alydon left a case of the anti-radiation drugs the Thals used to survive in Skaro's now toxic atmosphere as a token of peace for the strangers. He later met and befriended the youngest member of the party, Susan. He learned from her what the Dals had become in the centuries since the Thals had last encountered them, the creatures who now called themselves Daleks.
When the Daleks learned that their ancient enemies still existed, they used Susan to set a trap for them. They forced her to write a message which was left on the outskirts of the city for the Thal scouts to find, promising the Thals food. Despite Alydon's reservations, Temmosus enthusiastically accepted the Daleks' invitation. He led the Thals into the Dalek city, believing that a new era of peace and cooperation with the Daleks awaited them.
Temmosus marched with the Thals into the hall prepared for them by the Daleks, laden with food, curious as to why none of the Daleks were present but still suspecting nothing. The Daleks were hiding in the nearby alcoves, and as the Thals filled their bags with food, the Daleks emerged and ambushed them. Temmosus was the first Thal upon which the Daleks fired, even as he still pleaded for peace. (TV: The Daleks)