Lieutenant Michael Trent of the First Dragoons, U.S. Army, has a reputation as a man who will take any dare, fight any foe, and charm any woman. The son of an irresponsible man‑about‑town who eventually deserted his family, Michael sees himself following in his father's footloose steps. His commanding officer sets a stumbling block in his path, however, when he arranges for Michael to marry a lovely English immigrant. It's true flame‑ haired Ellen Winthrop is a beauty capable of inspiring heated fantasies, but Michael never saw himself as the kind of man who was fit to be husband and father. For once, he's faced with a challenge he doesn't know how to handle; if he doesn't watch his step, he could end up losing his heart.

When Ellen Winthrop learns the only way to secure passage to her uncle in California is to marry a soldier and travel across the Southwest with the Army, she agrees to wed Michael. The illegitimate daughter of a English Lord and a seamstress, Ellen has spent her life trying to blend into the background at her father's castle, despite an inner longing to be more daring. After a lifetime immersed in the petty gossiping and minor power struggles of life in her father's castle, Ellen thinks this brash, easygoing, and very handsome Texan may be just what she needs to make life more exciting. As Michael teaches her to be more adventurous, she finds herself wishing the marriage was more than temporary. Is she brave enough to risk all for love?

Set from San Antonio to Santa Fe in 1848, San Antonio Rogue follows Michael and Ellen on a series of adventures which brings them to a better understanding of each other and the powerful love that develops between them.

Michael quickly learns his assumptions about the ladylike Ellen aren't all true. He expects her to be shocked by stories of his past escapades; instead, she finds the tales exciting. He thinks the arrival of a group of camp followers at their wedding will upset Ellen, instead she welcomes them all and tells Michael he knows such interesting people. The outwardly demure Ellen has a spark of daring in her that intrigues him, making it all the more difficult for him to keep his pledge to maintain a purely platonic relationship with her.

With all the fanfare of a grand march, the troops leave San Antonio, headed west for California. Ellen rides with the other women near the baggage wagons and admires Michael as he leads his men. She likes the way he stands up to troublemakers, including Gregory Dunleavy, a Texas Ranger who joins their caravan. A brash man used to taking whatever he wants, Dunleavy decides he wants Ellen, and sets about winning her by fair means or foul.

The days spent traveling and nights sleeping in the open allow Michael and Ellen the chance to get to know each other better without the almost impossible temptation of life in close quarters.

Michael teaches Ellen to play cards, while she entertains him and her new friends in the company by showing off a few English country dance steps. Meanwhile, Dunleavy pursues Ellen, arousing Michael's jealousy. Michael doesn't trust Dunleavy and fears sweet Ellen will fall for the man's brash talk of a rich family estate in Scotland and a sprawling plantation in Texas. What does he have to offer in comparison? He's never stayed in one place long enough to accumulate property; before now, he never thought that it mattered.

Seeking to goad Michael into a foolish move, Dunleavy makes a wager that he can beat Michael at a game of cards. When another player catches the Scotsman cheating, he explodes in a fit of temper and threatens that player. Michael saves the day, but earns Dunleavy's true hatred.

The company stops for a few days' rest in El Paso. Once again, Ellen sets up housekeeping in Michael's tent and the couple spends time alone. He battles his desire for her, while she searches for a way to let him know she would welcome him into her bed. A lifetime of keeping silent about her own wishes hasn't prepared her to speak plainly about what she wants, but her efforts to help one of the camp followers instigate a romance with the Army doctor give her a few ideas.

Patrolling the streets one evening in an attempt to avoid going home to Ellen and temptation, Michael comes upon Dunleavy abusing a local storekeeper. The two men fight. Michael is the victor, but Dunleavy plots revenge.

The troops move on toward Santa Fe. The next stop after Santa Fe will be California. As the time nears when Michael will be free of  his obligation to her, Ellen longs for a more permanent union with  him. She decides to seduce him. Drawing on the advice of her friend among the camp followers, she boldly lures Michael with an invitation he can't refuse. After a night of passion, they confess their love for one another. Ellen believes they will live happily ever after.

But upon reaching Santa Fe, bad news awaits. Dunleavy accuses Michael of assault on a superior officer, an offense punishable by hanging. As Michael looks for a way out of this desperate situation, he decides he was foolish to think he and Ellen could ever enjoy permanent happiness. He can't seem to keep out of trouble and is sure before long she will lose patience with him, as his mother did with his father.

Ellen visits Michael in jail, determined to secure his freedom. She agrees to help him with a plan to win his release. When a visiting general inspects the troops, Michael shows up, shackles polished to gleaming silver, the picture of the perfect soldier, and boldly asks for clemency. Impressed by his boldness, (and decidedly unimpressed with ranger captain Dunleavy) the general grants him a pardon.

Ellen thinks everything will be fine now, but instead of returning to their tent, Michael moves in with a friend. He refuses to listen when Ellen tells him they are perfect for one another.

Meanwhile, Michael tracks down Dunleavy and promises to reveal all the ranger's dirty tricks to a group of Bandidos whom Dunleavy cheated out of their loot. In exchange for Michael's silence, Dunleavy agrees to leave.

Ellen, aggravated that Michael is being so stubborn, decides the time has come for bold action. Appealing to Michael's natural chivalry, she pretends to lose herself while searching for her wandering dog. When Michael races to her rescue, he finds her in a remote hideaway. Her bags packed, she announces she intends to strike out for the California gold fields, and the adventure she's secretly longed for all her life. She's tired of her demure, boring life and ready for action. If he thinks he's man enough to handle her, he can come along. Michael is stunned, then charmed, then aroused by this side of Ellen, a side he has only glimpsed before. For the first time, he begins to see himself through her eyes: as a brave, though sometimes foolhardy man with a strong sense of right and wrong and the ability to be faithful to the right woman. He realizes that losing her would devastate him. They make love, and make plans for a happy future together.