Kappatoo (1990). S01 E05 = "TIME-FUSE" [review]

"Simon's parents speak to Simon, not knowing he is Kappatoo, about the party, the hole in the front garden and the broken vase, as well as his strange behaviour over the past couple of days. But they are relieved that he is ok and suggest he comes with them to church so they can keep an eye on him. Outside the church, Kappatoo sees Tracy with Gregory, a popular boy at their school, whom she asks out on a date. Simon's teacher makes a visit to speak to Simon's parents about how well Simon has done in the computer test at school. Lucy and Tracy’s sister Sharon, after reading a book about aliens from Alpha Centauri, believes that Simon has been taken over by an alien from there, which leads to Lucy throwing a glass of water over Kappatoo, believing he will melt away as the character in the book she and Sharon were reading did.

Simon attends the trial at the court and answers Sigmasix’s questions wearing the truth detector in front of the judges. Sigmasix asks three questions to Kappatoo about cheating in The Hypergrid, but because it is Simon wearing the truth detector, not Kappatoo, and by switching places in time, Kappatoo cheated and not Simon, the truth detector states that Kappatoo is telling the truth. Kappatoo is acquitted and the judges depart. Outraged, Sigmasix realises this must not be Kappatoo and asks who it is while Simon is still wearing the truth detector. Simon replies his name and Sigmasix departs to find out where Simon is from. After his computer cannot find anybody with that name on the planet, Sigmasix threatens the computer that it will be history if it cannot locate him. Then Sigmasix realises that Simon is from the past." - from Wikipedia.