A motorist suffered a serious head injury following a road traffic collision on Leighton Road, near Northall on Tuesday night.
Thames Valley Police is appealing for information after the crash at approximately 9.50pm.
A blue Ford Fiesta was travelling on Leighton Road from the direction of Leighton Buzzard, towards Northall.
It left the road to the right hand side and came to rest in a ditch at the side of the road.
The driver of the vehicle, a 24-year-old man, sustained a serious head injury and was taken to the Luton and Dunstable Hospital for treatment.
The two passengers in the car, two 20-year-old women, sustained minor injuries and were also taken to the Luton and Dunstable Hospital for treatment.
Police would like to speak to anyone who may have witnessed this collision or seen the blue Ford Fiesta driving in the area around this time.
Call Thames Valley Police on 101 and ask to speak to PC Ian Stevens of the Milton Keynes Roads Policing Team.
Alternatively contact Crimestoppers anonymously on 0800 555 111.