In this scenario the object is to shoot down saucers using the Canons. It states that "To destroy a flying saucer, a Hero must use the "Fire the Cannon" card while in a Cannon space (paying 1 power token as normal)."
The Fire The Canon card reads " Play on a Hero in a Canon space with no Martians present. That Hero may pay 1 Power Token to Fire the Canon as a Ranged Attack."
We played it that as long as any Hero was on an undefended Canon space another hero (or himself) could play the card to allow him to Fire the Canon since this is how the card reads.
Another opinion could be that the scenario requires the person with the card in hand to be on a Canon space. Keep in mind this a Event card and Event cards can be played on other heroes.
Did we play it right or wrong or is it up to interpretation?