Space-General Sir Robert Simmons was a hero in the war between humans and the Omwanar. When it came time to sign a peace treaty, he was chosen to represent the humans as he was the only one trusted by the Omwanar.
However, before he could sign the treaty, he died of a heart attack. The First Doctor examined him and determined that he was killed by Space-Colonel Rufus Malvin. (PROSE: Making History)