Leonardo Da Vinci's quest to get to America really gets cooking tonight as he has to build a submarine to board a vessel. The scale this show attempts is always impressive, even if the visual effects aren't always up to snuff.

The episode starts with Riario trying to get a boat. Wait, that's not Riario, that's Leonardo, doing a great Blake Ritson impression. Unfortunately his ruse is found out by Prince Alfonso, who wants to have a sword fight. Alfonso is Da Vinci's better with swords, so Leonardo makes a hasty retry after noticing there are slaves below who he throws a dagger before jumping off the boat. The Pope is commissioning art when he's informed that Lorenzo has retaken Florence and that Cardinal Orsini has been hanged, which gives him an excuse to excommunicate the city.

Leonardo licks his wounds with Amerigo and Zoroaster nearby, and tries to find if there's another ship besides the Sentinel. It turns out there isn't one. Da Vinci comes up with half a plan: If they can get to the ship and free the slaves they can take it, but first they need to get on the ship. Amerigo sees a wanted sign for Da Vinci, which he gives to the artist. Leonardo does a better drawing of himself, and when he sees some crabs, an octopus, a turtle and a lamprey he gets an idea for an invention.

Lorenzo is facing the consequences of excommunication, so he plots to go to Naples to plead his case and leaves his wife in charge. An assassin perches on a rooftop and shines a blade. He communicates with someone in a castle. The assassin then takes a shot at the Cardinal Lupo Mercuri, which allows Lucrezia to capture him. She tasks Lupo with talking to the mystery man in the prison. As Amerigo reveals his business ruthlessness, Leonardo starts building his solution: A submarine.

Lorenzo and Clarice have a moment to be honest with each other, and she doesn't like his plan, and notes his mistress Lucrezia was a spy. But he won't be bullied and promises to save Florence before he takes off in disguise. Da Vinci, having finished his sub, wheels it out to sea and plots his course. Air is going to be their biggest problem. Lupo breaks into the jail and kills the guard. The prisoner reveals himself to be "the one true hope," the real Pope. On the Sentinel, Amerigo sells out Leonardo to Alfonso, while Alfonso looks for Da Vinci's discarded blade. He goes into the slave's cell and asks for the dagger back when a slave named Adana tries to kill him with it. Alfonso renames her "Toy" and has her sent to his chambers. Lupo finds out that the real Pope has been in prison has been in jail while his brother has been running things. The real Pope also gives Lupo a test to find out why the fake Pope took the name Sixtus, as Lupo knows the real answer.

Though Leonardo calculated all the details of their route, Zoroaster sees a wrecked ship which could crash them, so they have to paddle harder, which will take more of their air. They miss the ship and make it under the Sentinel and clamp on, but now they've got more leaks and way less air. Clarice is about to have dinner when a black man shows up claiming to be Carlo de Medici. She has doubts that he's really a Medici, but he knows of a secret room and many details about the castle. Lupo confronts Sixtus and asks about his name, and the Pope reveals he's an imposter. Cue shock music.

Leonardo and Zoroaster are waiting as they're running out of oxygen, which leads Leo to hallucinating about Lucrezia and Al-Rahim. Zoroaster gets them out as Amerigo starts a fire and throws them a ladder. Da Vinci goes to the slaves and asks them to fight with him for their freedom, but he needs to stall as they unchain everyone and so Leo is tasked with finding Alfonso and fighting him, even though he knows that Alfonso is the better swordsman. Alfonso is raping his "Toy" in his quarters when Leo busts in, and the two move out to the deck to fight. After a bit Leonardo is disarmed when he notes that Alfonso's lost. Alfonso doesn't believe it until he's disarmed by the slaves. Leonardo wants the ship deeded to him, which happens, and Adana gives Alfonso a nasty scar that spells "toy."

Lupo goes to meet Lucrezia now knowing the truth. He asks how she knew, and she reveals she's Sixtus' daughter. She asks to see her father. Before being sent off the ship, Alfonso threatens Da Vinci, who rubs in that Alfonso lost. Leonardo then tells the slaves they can leave at the next port, but he'd like them to come along, and promises them half of what they find. As they set sail, Leonardo sees Al-Rahim, who notes that Leo -- having deviated from the plan -- may end up killing those who help him. Though he's conversing with himself, Al-Rahim reveals that his plan will either lead to enlightenment or loss.

The Pope's a fraud! Florence is excommunicated! Leonardo has a swordfight! Submarine! Tonight's episode of 'Da Vinci's Demons' packed in a lot of stuff, and though my biggest concern going forward is how all these plot threads map out over the rest of the season, this was a big and exciting episode, and easily the best of the season so far. Though immeasurably aided by digital effects, the scale of what they're after here is pretty amazing, and though some cast members are left with a moment or two now that they're not dealing directly with Leonardo, this was a propulsive episode.

As the third chapter in an ten part series, this felt like a good gear shift into the further adventures of these characters. Tom Riley also got to shine in this episode in a way he hasn't been able to for much of the season. Here, besides his excellent Ritson impression, he revisits the charming egomaniac that was in the forefront of the first season, and his arrogance at the end of the fight was a great return to form. 'Da Vinci's Demons' seems more in line with serials than the great dramas of the day, but at the end of every episode, the measure of success should be if it makes you want to watch the next episode right away. By that standard, tonight's episode was a smashing success.