At Elysian Hall, a grand exhibition space in post-War London, a cast has been assembled for a medieval-themed pageant show replete with knights in coloured armour, real horses, and a damsel in a rickety tower on high. With death threats discovered by members of the troupe before the show, the worst comes to pass when the leading lady is thrown from the tower before the eyes of the audience by an unknown assailant – with all doors backstage also under observation. Faced with a seemingly impossible case, the wizened Inspector Cockrill and the fresh-faced Inspector Charlesworth begrudgingly join forces to uncover the killer hiding in plain sight.