William Shatner's A Twist in the Tale - Plot Synopsis: The Duellists

Two hundred years ago Tobias and Samuel both lost their lives in a bid to win the hand of Ariadne - who loved neither. Now, once a year, on the anniversary of the tragedy, the ritual of hate is played out again, and the fruitless battle waged with Ariadne once more helpless to avert it. Always before the duel has been unobserved; but this year things are about to change ...

The ghosts have always been bound to the site of the oak tree itself, but when they are seen, for the very first time, by a living person, they find that they are able to leave it. Samuel has an idea: if they could possess real bodies once more, they could settle the feud once and for all. Ariadne is alarmed by the implications, but Samuel and Tobias don’t wait around to listen to her objections - they melt into the air in hot pursuit of Ben.
Tobias makes the first move - and finds that he can indeed infiltrate another human being, though it’s tough work. (Ben - bewildered by the momentary takeover - would no doubt agree.) It’s settled, then: the duellists will choose appropriate hosts, and using the humans as instruments, duel for Ariadne for the final time. The situation leaves Ariadne with no alternative but to comply, despite her misgivings: unless she chooses a corporeal form for herself, she will be powerless to avert another pointless tragedy. Having followed Ben to school, the interlopers look around them. Samuel chooses Darren as his victim, and Ariadne, Zoë.

So the two duellists are now embodied in Ben and Zoë - and the sweet feminine mediator, in the hulking bully Darren. The scene is set for some ludicrous mismatches - and great bewilderment for the unwitting human hosts - until the children can figure out what is happening and how to take control of their own bodies again ...
... But the only way to achieve that is to effect a reconciliation between the ghosts - and it takes the very real danger of losing both Darren and Ariadne forever before Samuel and Tobias come to their senses, and in setting the children free once more, free themselves.