Part of the Ice Equation attached itself to him, encouraging him to bring of the rest of it through to Earth. He made Patrick Unwin work on equations that would help. When he felt Unwin was no longer useful, he killed Unwin.
Later, the Equation killed and siphoned off energy from him so it could attack the Seventh Doctor. A portion of Brett survived and was used to attempt and energy transfer from the TARDIS to the Equation's dimension, but was stopped by Adrian Molecross. (PROSE: The Algebra of Ice)