The road starts in two places on the B640 in Oakham town centre. The mainline starts at a mini-roundabout at the eastern end of the High Street and heads north along Burley Road. Meanwhile the B668 also leaves the B640 by the level crossing and follows Station Road past the station to give way to the mainline. Burley Road then continues on to cross the A606 bypass at a roundabout before climbing gently to reach the small village of Burley.
The road runs along the top of the ridge to Cottesmore, where it runs along Main Street before bearing sharply right on reaching a road leading to Kendrew Barracks (the former RAF Cottesmore). There's a sharp left-hand bend soon afterwards and then we leave the village and start descending to reach the next village, Greetham, whose main street seems cramped. At the far end of the village we cross a stream before following the dead-straight Stretton Road for a while. After we start bending again we reach the A1. The road has been extended here as part of that road's grade separation and so we meet the northbound offslip before heading north alongside the A1 for a couple of hundred yards past the back entrance to the Ram Jam Inn to reach a roundabout. The northbound onslip leaves here and we turn right to go under the A1 and end at the next roundabout, which provides access to the southbound sliproads and a minor road into Stretton.
The B668 was originally much longer, and started on the A6 in Kettering, heading north via Corby and Uppingham to reach Oakham. However, the route as far as Oakham was soon upgraded as an extension of the A6003.
Bypasses in more recent years have seen the ex-B668 in the centre of Kettering being renumbered to the A6900 and in Oakham to B641. The road has also gained a spur in Oakham, Station Road, that was originally the B673.