Theobald I was the son of Theobald le Vieux of Blois, who from 908 on was viscomte of Tours. His wife, and the mother of Theobald was Richildis, a great-granddaughter of Rorgon I, Count of Maine. Theobald I was initially a vassal of Hugh the Great, Duke of France. Around 945, Louis IV was captured by Northmen and given over to Hugh the Great, who placed the king in Theobald's custody. After about a year in his vassal's custody king Louis negotiated his freedom by offering Hugh the city of Laon which Hugh then gave to Theobald. Theobald was the count of Tours from 942, was also count of Blois and in 960 count of Châteaudun and Chartres.
Theobald's sister had married Alan II of Nantes, the Duke of Brittany, giving Theobald influence all the way to Rennes. However the death of Alan II left a void in Brittany and left it vulnerable to encroachment by either the Normans or the Angevins. Theobald made an alliance with Fulk II of Anjou which gave him control of Saumur a strategic citadel located between the Loire and Thouet rivers guarding the Angevin march. This included control of the monastery of Saint-Florent inside the walls of Saumur. In turn the recently widowed Fulk married Theobald's sister, the widow of Alan II of Nantes.
In 960, he began opposing Richard I of Normandy and entered into a long war with the Normans. In 961, he attacked Évreux. The Normans responded by attacking Dunois. In 962, he launched an assault on Rouen which failed. The Normans burned Chartres in response. He took control of the fortresses of Saint-Aignan in the Loir-et-Cher, Vierzon, and Anguillon in Berry. During the minority of Hugh Capet, he reinforced Chartres and Châteaudun. By his death, he had built a vast power on the Loire, dominating central France.
About 943-44, he married Luitgarde of Vermandois, widow of William I of Normandy. She was the daughter of Herbert II, Count of Vermandois and Hildebrand of France, daughter of king Robert I of France. Her half-brother was Hugh the Great Duke of France (wikipedia.org)