(this image appears for illustrative purposes only and no attempt is made to supersede any copyright attributed to it)
Cobb was a general encountered by the Tenth Doctor, Donna and later Martha on the planet Messaline. He commanded Cline and the Doctor's daughter, Jenny.
When Cline captured the Doctor and Donna, he took them to see Cobb, who assumed they, along with Jenny, were pacifists from the Eastern Zone.
Cobb asked the Doctor to join his army in the war against the alien Hath. He sought the Source. He believed the Source would give him the power to kill every Hath on the planet. When the Doctor refused to help and instead spoke of stopping him, Cobb had the Doctor, Donna and Jenny arrested for not supporting him in the war. Cobb later found Cline tied up on the floor of the cell, discovering that the three of them had broken out. Cline said Jenny had tricked him.
Cobb and his soldiers chased the Doctor, Donna and Jenny into secret passageways uncovered by the Doctor. When Jenny tried to buy them time by holding up the soldiers, she hesitated to kill Cobb. He tried to convince her to join his cause, stating that it was "in [her] blood". Jenny was already loyal to her father, the Doctor, and he had told her there was always a choice in killing someone. Instead, she hit a pipe to delay Cobb and his troops. When the Doctor told Cobb that if the Source was a weapon he'd never let him have it, Cobb threatened to kill him.
The Doctor, Donna and Jenny were reunited with Martha and found the Source first. They discovered it was a terraforming device. The Doctor declared the war over and released the terraforming device. Cobb blamed the Doctor for ruining his chance at victory and tried to shoot him. Jenny stood in the way to protect her father so Cobb shot her. She was shot through one of her hearts and temporarily killed. The Doctor held a gun to Cobb's head and contemplated killing him, then stopped, telling Cobb that "[he] never would" and that it was on that basis that the new society of human and Hath should he founded. Cobb was demoted, arrested and imprisoned for Jenny's murder.