Two 21-year-olds were threatened with a knife as they walked their dog on Wednesday evening.
The couple were approached by two men who demanded they hand over their belongings a footpath between Shenley Brook End and Furzton, near to Egerton Gate, between about 6pm and 6.15pm.
One of the victims was then chased by the offenders V3 Fulmer Street between Shenley Brook End and Shenley Lodge, where he managed to speak to a passing motorist.
The offenders then fled the scene on foot.
Nothing was stolen during the incident and the victims were not injured.
One of the offenders is black, in his early twenties, about 6ft 2ins, with short hair.
He was wearing a green and black t shirt.
PC Caroline Vendy said: “I would like to hear from anyone who recognises the description of the offender provided, or saw anything suspicous in the area at the time.
“I would particularly like to hear from a motorist who spoke to one of the victims. The motorist was driving a Nissan Juke vehicle.
“If anyone has any information, no matter how insignificant they believe it might be, please call police via 101.
“If you don’t want to speak directly to the police you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or online at www.crimestoppers-uk.org.
“No personal details are taken, information is not traced or recorded and you will not go to court.”