Late for school, Paul and Fiona take a short cut past an old castle and hear the shouts of someone in trouble. In the cellars, they rescue a man trapped underneath some strange electronic contraption. Introducing himself as Professor Wagstaff, he explains this is his time travel device, OSKA (Oscillating Shortwave Kinetic Amplifier). Invited to return later, the children head for school. Paul is late for 'Sniffy' Kemp's history class, earning a ban from Miss Campbell's school play of Robin Hood.

At lunchtime, Paul and Fiona agree to help test OSKA - Wagstaff gives them a recall belt that will return them to the present. The test appears unsuccessful, but returning to school that afternoon, they find it's morning again.

In Robin Hood costumes, Paul and Fiona plan a trip to 1190, but when OSKA blows a fuse they arrive in 1953, to meet an overbearing schoolboy version of Sniffy. A teacher intervenes and, finding Fiona's name is Hatton-Jones, takes her to meet her grandpa, now much younger and with no idea of who she is. Explanations are avoided when Wagstaff patches up OSKA and shoots them back further in time.

Unable to control OSKA, Wagstaff sends the children to various times, where they encounter assorted Sniffy ancestors - a duelling nobleman about to shoot a Hatton-Jones, a traitorous Cavalier, a caveman. They escape the present, avoiding a Stone Age bear.

Undeterred, Fiona tries a trip to 1635, but OSKA malfunctions, sending her to 1941, where she meets a young Miss Campbell and her grandfather in an air raid. An explosion sends Fiona hurtling around various eras - eventually she's captured by a Highwayman Kemp and imprisoned in the castle dungeon. Paul zaps back to find Fiona and they return safely.

Now ready for time-travel himself, Wagstaff dresses as a court jester, set for 1364. With Paul operating OSKA, Wagstaff travels to the court of a king who is less than amused by his act and orders the professor to be beheaded. Wagstaff's recall belt has stopped working as Sniffy has tracked Paul down and, suspecting him of running a bomb factory, delivered him to the police. Fiona's lacrosse team make a timely intervention by storming the castle; in the confusion Sniffy is 'reverted' to being a schoolboy. Unfortunately Paul, Fiona and the Professor fall into the machine and the children suddenly find themselves back at the very moment they first met Wagstaff.