William Shatner's A Twist in the Tale - Plot Synopsis: A Matter of Time

A mysterious newcomer to Alverton seems to know and understand more about David, his work and his problems than anyone else. What does the middleaged, balding and paunchy Donald Wells have to do with young David’s life? When Alan’s shop is broken into and the prototype Tasmo stolen, Donald seems the obvious suspect.
But Donald Wells is from the future - he’s been sent to ensure David does get the Tasmo back, because without it the development of time travel is impossible. He meets David "accidentally" in the park, and drops a clue as to the true identity of the thief that David can barely credit. In his groundbreaking work on the Tasmo, David has miscalculated more than the chronology determinator: dowdy Miss Jameson is not - or not simply - the altruistic mentor she seems. While essentially good-hearted, she has fallen desperately in love with the school headmaster, the cold and manipulative Eamonn Dodds, and at his urging has entered into a money-making scheme to get them out of Alverton forever - together. The Tasmo is a hair’s-breadth away from actually working - and the plan is to steal it from David and sell it to the highest bidder.

This isn’t the future as David imagined it - and if rescuing the Tasmo means that David will grow into the balding and overweight Donald, then it’s time to change the course of history! David runs away, leaving Donald behind to be caught in the school grounds and arrested for breaking and entering - but not before he has hidden the Tasmo.
Donald has had a pointed conversation or two with Alan, too - and when Alan realises that David has abruptly lost all interest in time travel and turned overnight into a superjock wannabe, he is more anxious than gratified. David refuses to discuss his change of heart, leaving his father very worried ... When he sees David being shepherded into the headmaster’s car and driven away, his concern turns to fear.

But Alan remains resolute under fire: he tells David he must do what he wants with his life, not what Alan or Dodds or Miss Jameson want ... Dodds sneers, but he would have done better to have kept alert: the moment she sees he is distracted, Miss Jameson creeps up behind her erstwhile suitor and knocks him unconscious with a vase. As Dodds falls, the vase shatters - and there in the shards lies the Tasmo, David’s historic invention - and his future.