The Double-Dealer is built around the schemes of a man named Maskwell who attempts to marry another man's bride and become the heir to an estate that he has no claim to.

When Maskwell finds out that Mellefont is in love with Cynthia and hoping to marry her, he decides to ruin his reputation. He convinces Cynthia's father's wife that Mellefont actually wants to sleep with her. He convinces Mellefont's uncle that Mellefont is sleeping with his wife, Lady Touchwood.

Ultimately, Cynthia's parents see Mellefont as unsuitable, even though her father's wife wants to sleep with Mellefont. She believes he only wants to marry Cynthia to get to her. Lord Touchwood names Maskwell as his heir instead of Mellefont.

However, ultimately, Mellefont's friend Careless steps in and helps people see the truth. Cynthia and Lord Touchwood overhear Lady Touchwood admit her guilt in trying to make Maskwell look bad for rejecting her when she tried to sleep with him. He disguises himself and hears her try to make another attempt at being with Mellefont. This clears things up. Maskwell is exposed as a fraud and Mellefont and Cynthia are able to marry.

The plot of The Double Dealer largely concerns two innocent and sincere characters, Mellefont and Cynthia, trapped in a labyrinth of deception and evil machinations created by the jealous Lady Touchwood and the manipulative Maskwell.

Mellefont, the heir apparent of Lord Touchwood, seeks to marry Cynthia, the daughter of Sir Paul Plyant. Lady Touchwood, who is in love with Mellefont, aims to ruin the match. She enlists the help of Maskwell, her lover, to tarnish Mellefont's esteem in Lord Touchwood's eyes. To this end, Maskwell convinces both Lords Touchwood and Plyant that Mellefont is involved with their respective wives.

Mellefont is bereaved of his inheritance and Cynthia is to be married to Maskwell. However, when Lady Touchwood discovers this arrangement, it awakens her jealousy. The ensuing confrontation between Lady Touchwood and Maskwell is overheard by Lord Touchwood, who thwarts Maskwell's plan.