The road started on the A604 in Thrapston and headed south along the River Nene to Denford before climbing out of the valley and crossing a ridge to pass through Ringstead. After meeting the B666 to the northwest of Raunds the road skirted the western side of the village to end on the B663.
The B665 was upgraded to become part of the A605 before 1930 when that road was extended south of Thrapston to meet the A6 near Higham Ferrers, making the B665 number redundant.
The entire road has now been bypassed by the current route of the A45. Except for a hundred yards or so on the northern edge of Raunds, which is part of the B663, the ex-B665 is unclassified.