When, at the age of 35, the wealthy Earl of Rule proposes for the hand of Elizabeth Winwood, she resigns herself to marrying in order to rescue the fortunes of her family, impoverished by gambling.

Before the game can start, Rule manages to pitch Lord Robert into an ornamental pool and dresses himself in Lethbridge's mask and domino.

When Rule continues the masquerade by stealing a kiss as he snips off her hair, Horatia rushes out in fury and bumps into Lady Massey, who is at the same ball.

When she leaves a ball early, Lord Lethbridge arranges to kidnap her to his own house, intending to ruin her and so gain his revenge on Rule.

While she was working on The Convenient Marriage, the author sent a "Dramatis personae" of its main characters to her agent, L. P. Moore, in the course of their correspondence: According to Heyer's biographer Jennifer Kloester, the creation of such cast lists often served as the starting point in plotting her novels.

Once she had established a suitably named character, she found it easy to create their individual voices and move the action forward through their lively dialogue and interaction.

So, on the basis of the list above, Heyer speculated in her correspondence with Moore, "All these people are naturally going to fall into a number of awkward situations, & I rather think Pelham has a duel with friend Crosby, while I am quite sure that Rule has one with Lethbridge.

[2] Certainly Heyer's return to similar stock situations later in her career led to A.S. Byatt's accusation that April Lady (1957) was no more than "a rehash of the earlier Convenient Marriage".