In 1843, a group of teachers are held as political prisoners at Diablo Corozan, an island 40 miles off the coast of Tampico, Mexico. Among them is Justin Marchand and his daughter, Joan. Joan insists on trying to escape and the other prisoners override Justin's objections and help her. As she climbs out, the prisoners are forced to gag Justin when he tries to call the guards. Joan escapes on a horse but then rides into the woods and fakes being thrown. When two soldiers, Ramone and Carlito, catch up to her, they haul her back to the prison.
In New Orleans, the Enterprise is loading up for an easy cruise to the town of Piquano. Grey is leaving Chip at a boarding school to get a few weeks of an education while they're gone. As Carney and the ship's new cook, Pickalong, go ashore for an hour, Joan brushes past them and tells Grey that she wants to book passage to Piquano. Grey quickly realizes that she doesn't have any money and Joan admits that she needs it for an important mission. The captain isn't interested in having another scheming woman onboard and sends her ashore. As she goes with Carney, he assures her that Grey isn't a bad sort. She begs the engineer to help her and he agrees to get her some food and take her to Ben, who is relaxing in town.
At a saloon, Joan tells Ben that Justin and the other teachers traveled to Tampico to help educate the locals so they could rule themselves after they threw out the Spaniards. As they talk, Joshua comes by and tells them that Grey is preparing to cast off. Ben tells him to wait and Joan continues, explaining that when the guards took her to the commandant, she offered to return to the States and raise $10,000 to pay as ransom for the release of her friends. However, Joan tells Ben that she plans to use the money she raised to buy guns and men and then charter a boat to return to Tampico and free the island. Ben, touched, tells Joan that he'll convince Grey to take her to Piquano.
When Ben and the others return to the Enterprise, Ben tells Grey Joan's story. The captain cuts him off and tells the crew to cast off. Ben apologizes to Joan, admitting that Grey has recently been burned by women coming aboard, and Joan thanks him for trying. She then walks off the ship onto the dock and picks a fight with a worker. The Enterprise crew comes to help her and the dock workers fight back. Grey strides casually through the melee looking for Ben. When he fails, he yells fire and everyone breaks it up. Grey looks up and sees Ben watching the fight from the pilothouse. The pilot says that he has no idea where Joan is and Grey looks at him skeptically before ordering the crew to cast off.
That night on the river, Pickalong brings a meal to the captain's cabin. Grey notices that the blackberries they recently purchased aren't there and the ration is too small, and Pickalong claims he has no idea where they went. The captain wonders why Pickalong is clutching at his hat and the cook says that he has a headache.
Ben goes to see Joan in a storage closet and she gives him a kiss on a cheek. Ben assures her that he's helping her because it's right and doesn't want any kind of reward. Pickalong arrives with food, including the blackberries that he hid under his hat, and has his monkey Petey stand watch outside. The cook then gets out his guitar and plays Joan a song.
Grey is going to the galley to get more food when he notices a smiling Carney walking by with a pillow. The captain follows him and sees him go into the storage closet. They hear Grey approaching and quickly hide Joan. The captain barges in and is surprised to see the three men huddled in a closet, singing to Pickalong's music. He notices a discarded plate with chicken bones and blackberry juice, and Carney claims that he was going to sleep there for the night. When Joan realizes that Grey is onto her, she storms out and defends Ben for helping her. Grey tells Ben to take the boat ashore so they can put Joan off, but he refuses.
Rather than face a mutiny, Grey takes Joan to his cabin and listens to her story. He accepts that she's telling the truth and wonders how she convinced Ben, and Joan demonstrates by flirting with him. Grey calls her bluff and kisses her, and she slaps him and insists that she isn't that kind of girl. Joan switches to begging and Grey agrees. However, he points out that Piquano had a bad harvest and she won't find many donors. He does offer to convince the casino owner there to hold a benefit in return for 50% of the take. Joan hugs him in relief and wonders how he knows she isn't lying, and Grey finally tells her that he and Justin are old friends and the Enterprise was the ship that took the teachers to Piquano.
The ship shakes as Ben unexpectedly docks the Enterprise at Cornell Bend. Grey figures that Joan is somehow responsible and drags her up to the pilothouse. Joshua is there and tells the captain that Ben, Carney, and Pickalong have gone ashore to raise money. In town, the three men are busy extorting money from the local shopkeepers, threatening to cut off their business if they don't donate money. As they come aboard, the shopkeepers summon the sheriff and the Enterprise crew quickly pull up the gangplank. When the sheriff tells them to give back the money, Ben ignores him and tells Joshua to head out.
Grey gathers with the men and they tell him that they're going to do the same thing in every town until they get to Piquano. He points out that the sheriff will send a rider ahead to the next town and all they've done is made sure they can't raise any more money. Grey orders Joshua to turn the ship around for New Orleans... and on to Tampico. Ben and the others object, pointing out that there's a hundred miles of open water with no chance to refuel. Grey takes the $800 they raised at Cornell Bend to buy fuel in New Orleans to make it to Tampico. When they wonder how they'll get back, Grey tells them that they'll cut the wood for fuel when they get to Tampico.
The Enterprise enters the Gulf of Mexico and heads for Tampico. Grey checks on Joshua at the wheel and the mate says that it's the second time he's taken a riverboat across the Gulf. Surprised, Grey points out that it's impossible and Joshua wonders why they're attempting it. The captain explains that they'll follow the coast rather than go across open water, and on the way back they'll pick up two thousand bales of hay at Galveston to make up for their losses.
As the Enterprise proceeds along the course, Joan brings some food up to Ben in the pilothouse. The smell triggers a bout of seasickness and he runs off, leaving Joan to man the wheel. Grey comes up and is surprised to see her steering the ship, and Joan insists on doing her fair share. She wonders how she can repay him and Grey says that he's doing it for the same reason that she is. Joan wonders if he has an ulterior motive and mentions the earlier kiss. Grey suggests that she might be interested in him and she takes offense. However, when he says that he isn't interested in her, Joan is even more offended.
A storm hits the ship and Pickalong crawls out on the bow to relieve the watch. Once he's settled in, a log chain snaps and the cords of fuel fall on the cook's leg, pinning him. The crew run to help their comrade and Grey goes out on a breeches buoy above the loose wood. Once he sends Pickalong back, Grey uses a new chain to secure the fuel and the crew pulls him back. As he steps down onto solid deck, Joan runs to him and hugs Grey in relief.
As the Enterprise approaches Diablo Corozan, Grey works out an approach into the bay. Joan is with him but is spending her time gazing at him and asking him questions about boat captain's getting married. She finally kisses him and Grey kisses her back. They're interrupted when Ben calls down, well aware that Joan is with Grey, and tells them that they're approaching the island. Grey tells him to have the crew dowse the lights and ready the guns.
Once the crew beaches the ship, they head for the prison. Joan points out where the guards are, and they're forced to take cover when Ramone goes out to investigate. Grey knifes him and impersonates Ramone's voice, telling Carlito that everything is okay. The crew then scales the walls and gets inside the compound. Ben kills Carlito and Joshua is forced to shoot down another guard who spots them.
As the remaining guards come running, Grey runs to kill the commandant but his gun jams. He grabs a sword and fights the commandant. The Spaniard manages to knock Grey down but the captain runs him through. Meanwhile, the crew disposes of the remaining guards and Joan runs to her father. They go to get Grey, who can't get up because he sprained his hip in the fall. Much to Grey's annoyance, Ben tells the crew to load their captain into a stretcher and take him to the ship, while Joan fawns over him, assuring Grey that she won't leave his side.