Running on the motherly intuition of Fralia Grey-Mane, the Dragonborn investigates the House of the Battle-Born's in Whiterun as to the whereabouts of her son. Finding a book on a bureau, the Dragonborn discovers that the Thalmor apprehended Thorald, suspecting him of working with the Stormcloaks. The Thalmor were holding him prisoner at Northwatch Keep. Alongside his brother, Avulstein, the Dragonborn heads to Northwatch Keep to rescue him.
After being rescued, he says he'd better leave Skyrim or join the Stormcloaks in order to evade the Thalmor. In his testimonial after leaving Northwatch Keep, he affirms that he's not sure about how much time has passed since he was abducted, but estimates it may be more than a year.
He asks the Dragonborn to tell his mother, Fralia, a secret message to be decrypted by her. After escaping the Keep, Thorald joins the Stormcloaks so as to take the fight to the Thalmor agents. Henceforward, he may be seen in Windhelm telling Ulfric's second-in-command, Galmar Stone-Fist, that he wishes to fight for the Stormcloaks.
During the Stormcloak's assault on Fort Amol, he may appear and help to capture the fortress.