Cartoon adventures of builder Bob and his friends - Travis the tractor, Scoop the bulldozer, Lofty the crane and Dizzy the cement mixer. Bob, Wendy and the Machines are spending the day doing jobs to make the town a nicer place in which to live. Lofty has a magnet attached to his crane so that he can pull up the pieces of track off the railway line. Mr Bentley looses his special pen, but Bob realises it must be under the turf that he has laid. How will they find it?