Two thugs, Lee (Mark Butt) and Ainsley (Christopher Brett), have kidnapped executive Mark (Lewis Marsh). Why, we do not know. Mark thinks this is an exercise planned by his employer to check safety procedures at the company. This is confirmed by Lee's sudden use of the word "harlequin", which is Mark's company's "safety word", indicating that this is not a real kidnap but a training exercise. Unfortunately for Mark, Lee meant to say "mannequin". This is for real, and blood will be shed before the night is out.
Unfortunately for the audience, the word "harlequin" is also literally the only connection between the play's title and the commedia dell'arte style of the play's opening, which is otherwise completely inexplicable. The remainder of Anthony Trevelyan's weakly derivative play neither references nor supports a commedia dell'arte style of theatre, and Catherine Phillips' direction does not attempt it. Butt and Brett are unconvincing and unthreatening thugs. Marsh (impressively, since he is the only cast member who cannot move at will, tied up as he is) does his level best to keep the tension up, but for a 50-minute chamber piece about a violently dramatic event, Harlequin is remarkably lacking in dramatic tension of any kind.