It started well for the Knights, capitalising on the Pirates going two men down to open the scoring when Jack Spittle stole the ball and made a fast-paced break, drawing in the final defender before handing off to Alex Dolly on his supporting line.
Sam Olver coverted but Knights were unable to build on the lead, and Tom Duncan and Tom Channon crashed over for the hosts to give them a 12-7 lead at the break.
Doncaster’s precision in execution was starting to let them down and it got worse after the restart as a fired-up Pirates team scored two tries in the first 11 minutes to put the result beyond doubt.
First, Channon spotted an opening to score his second and then Tom Kessell burst through a hole in the defence to give Cornish their fourth, bonus-point clinching try.
Pirates would score three more points via a penalty as Doncaster toiled to get a foothold in the contest but to no avail, meaning Pirates stay second, Knights fourth.