Edward the Confessor

 was King of England in the 11th century, one of the last Anglo-Saxon kings before the Norman Invasion. Both he and his Queen Edith of Wessex were considered shrewd rulers during a time of peace. The Eighth Doctor visited his court in 1055 and exposed a plot against the crown by Leofric of Essex.

Biography
When Edward was a child, the Eighth Doctor had visited the court of his father, Æthelred the Unready, and cured Æthelred of a heavy fever. Edward was greatly impressed and indebted to the Doctor.

In 1055, Bishop Leofric of Exeter presented the King and Queen with exquisitely crafted necklaces, made of a new metal that was warm to the touch. The Doctor arrived shortly afterward, to Edward's pleasure... and Edith's chagrin. The Doctor's interest (and Edith's suspicions) made the two of them contrive a ruse to get Leofric to expose his plan: kill the King and Queen with necklaces made of plutonium, and assault the castle with tons of the stuff in an attempt to summon the Nimon to Earth. With the Doctor's help, they defended the castle from attack, and disposed of the plutonium. The Doctor gave them anti-radiation pills to counteract the effects of the necklaces and assault. (AUDIO: Seasons of Fear)

The Doctor had also visited Queen Edith in her younger days, at the court of her father Godwin of Wessex. Edith and the Doctor engaged in deep and animated political discussions for a few years. She misinterpreted the Doctor's interest and proposed marriage. When the Doctor couldn't find a way to extricate himself from the arrangement, he fled in the TARDIS. Edith later married Edward the Confessor and became Queen of England, but harboured a grudge against the Doctor.

Edward's peace was largely due to his having no heirs, and to promising each of his rivals the kingdom upon his death — "Edward and his wife Edith promised the throne to William of Normandy, Harald Hardrada of Norway... everyone who wanted it. They kept them at each others' throats for decades!  Nobody wanted to invade, because they all thought they were going to inherit." (AUDIO: Seasons of Fear)

The Doctor blamed the "mad genetics" of the next few generations of royalty on radiation damage from the plutonium.