At Shangri-La Edit
When the Chimeron Delta arrived at the camp, Billy took a shine to her and sang "Why Do Fools Fall in Love" in dedication to her. She told him of the Bannermen and he helped her look after the Chimeron princess. He took Delta and the child to a secluded spot by a river where they rested. Ray and the Seventh Doctor warned them of the impending attack by the Bannermen.
Against the Bannermen Edit
The Doctor took Delta, Billy and the Chimeron princess to stay with Goronwy. While there, Billy stole some Chimeron jelly from Delta. She caught him eating it and he told her he wanted to become a Chimeron so that he could come with her.
Billy helped the Doctor set up his speaker system so that the Chimeron princess could send the Bannermen into disarray with her singing. The Bannermen were restrained and tied up aboard their craft. By this time Billy was fully transformed, and his skin had become a silvery colour. He made his goodbyes and told Ray to take care of the Vincent as he boarded the Bannerman craft.
Billy was oblivious to Ray's crush on him; he never noticed how she mimicked his actions and took his advice strictly.
He took to Delta when he saw her arrive at the camp and sat with her while she ate. He later dedicated a song to her and turned up at her dorm with flowers.
Billy was seemingly unfazed by the revelation that Delta was an alien and continued his advances. He took Delta to secluded beauty spots known only to him and Ray. He helped her care for the Chimeron Princess.
Billy stole Delta's Chimeron jelly when she was not looking and ingested it despite the fact it could harm him. Together with the Doctor, he risked Bannerman fire to set up the speaker system.
The Doctor described his decision to change as rash but recognised that love was not rational. The Doctor questioned him as to his decision but was unable to sway him from it. Before Billy left he told Ray he would always remember her being at Shangri-La. (TV: Delta and the Bannermen)