Robert Hay was a favourite of King James in 1605. The King's waning interest in him coincided with the arrival of the First Doctor and Vicki Pallister, who was disguised as "Victor", a boy. Hay seemed extremely jealous of the King's attention for Victor.
Hay was actually a master in a secret brotherhood of mystic sorcerers. He was playing both sides of the Gunpowder Plot, planning to take advantage of the chaos after Parliament was blown up. He expected the Brotherhood to fill the power vacuum, ruling England.
To this end, he used a number of disguises to spy on and deal with the various factions. Some of them included "the Spaniard" and "Sybil", a tavern maid.
Hay was going to set off the explosion under Parliament when Firking and Hodge, two cobbler friends of Ian Chesterton, found him as they were trying to rescue Ian. They overpowered Hay, then Sir Robert Cecil arrived and arrested him. Since the real Guy Fawkes had been killed, Cecil thought it politically expedient to charge and execute Hay under Fawkes' name. (PROSE: The Plotters)