Bob the Builder's dad comes to town and takes over the renovation of an old castle. Bob doesn't like taking a back seat and tries to tell his dad, but doesn't want to hurt his feelings.

Bob The Builder and his team gang of talking construction vehicles find out about the remains of an old ancient legendary medieval castle in Sunflower Valley, Bob's father, Rob The Builder, has come to help out also with the help of Doctor Florence Mountfitchet. join Bob and all his friends in a big medieval fantasy-adventure special with a-MAZE-ing mazes of doom and dark haunted dungeon prisons full of giant spiders, to fix up the castle to what it was in ancient early AD medieval times and get knighted into becoming "THE KNIGHTS OF CAN-A-LOT".