Bob is the fierce guard dog of Nuthanger Farm who proved to be an obstacle whenever the rabbits of Watership Down ventured there. Later, however, Bob unwittingly played a crucial role during General Woundwort's attack on the Watership Down Warren that ultimately resulted in a victory for the Watership Down rabbits, defeat for the Efrafans and the permanent disappearance of General Woundwort.
After being discovered by the farm cat, Tab, Bob awakens and upon seeing the rabbits, starts barking furiously, attracting the attention of the Nuthanger Farm humans. This was repeated during the next visit to the farm during which Hazel attempted to bring back the hutch rabbits to Watership Down, but fails.
During General Woundwort's attack on Watership Down, Fiver went into a trance, and started moaning about "a dog loose in the wood". From this, Hazel created a plan to release Bob and lure him to the attacking Efrafans that were attacking Watership Down. After Hazel released the dog, he was caught by the farm cat Tab, but saved by Lucy, one of the humans of Nuthanger Farm. Dandelion and Blackberry then continued the plan, leading Bob back to Watership Down where he attacked, mauled and threw the attacking Efrafan forces into disarray. He also attacked Woundwort, the outcome being ambiguous because Woundwort's body was never found. Bob returned to Nuthanger Farm with minor injuries, presumably caused by Woundwort after he apparently killed the Chief Rabbit in a violent fight.
In the Netflix series, Bob is a large, muscular Rottweiler with a few small, but noticeable scars.
Bob is first seen sleeping in his kennel when Hazel, Fiver and Bigwig come to rescue the hutch rabbits from Nuthanger farm. When they finally manage to get them out that night, Bob awakens and attempts to attack the rabbits, but is held back by his rope.
Later, Fiver bites through Bob's rope so Hazel and Blackavar can lead him to the Efrafan army. During the chase, Bob comes very close to catching Blackavar, but Hazel gets his attention at the last second and successfully leads him to the Efrafans. Bob scares off the Efrafans and manages to kill Captain Orchis who is paralyzed by fear. However, General Woundwort stands his ground, declaring that he fears nothing and leaps at the dog. Woundwort and Bob are last seen lunging at each other before their shadows fade, leaving both their fates unknown.