The episode begins with Annabeth venturing to an old hideout near the sewers. A flashback unfolds, revealing Annabeth and Luke concealing themselves there while Thalia battled a cyclops elsewhere. Luke pulls out a bracelet from a Kellogg's box and hands it to Annabeth, calling it a protection charm. Soon, Thalia arrives, swiftly followed by Grover, who informs them of Camp Half-Blood. In the present, Annabeth discovers the same bracelet in the hideout, along with other supplies.
Meanwhile, Tyson and Percy wait by a beach. Suddenly, their raft explodes. It's Clarisse. She demands that the trio reveal the real coordinates to the Sea of Monsters. Percy challenges her to a duel, during which Annabeth "kills" several of Clarisse's dead sailors – specifically those who can navigate the sea, run the engines, and know about the sea of monsters. This means Clarisse now needs the trio's unique skill set on board.
As they journey on, they discuss their path ahead. To reach the sea of monsters, they must choose between two monsters: Scylla (a tentacled monster sitting atop a cliff) or Charybdis (a whirlpool-creating monster under the sea). Clarisse prefers Charybdis, but Annabeth thinks it wiser to try Scylla. If her six tentacles take up six dead sailors, the rest can pass through. However, Clarisse has promised Elysium to the soldiers and doesn't want to betray them.
A flashback shows Grover leading Annabeth, Luke, and Thalia to a satyr hideout. Thalia and Luke want to go their separate ways, but Grover insists that camp will be much safer. In the present, Annabeth throws her bracelet into the sea, but a wave returns it towards the ship. Elsewhere, Percy realizes he instinctively knows the ship's speed and location. Later, Tyson finds Annabeth's bracelet on deck and returns it to her. She throws it back into the water, and we see a fin rise and dip back into the sea.
In the past, Annabeth sneaks out of the satyr hideout to search for the bracelet she lost. She encounters three cyclops who kidnap her. The others arrive in time to save her, but the incident changes Thalia's mind about Camp Half-Blood. As they near camp, Furies begin chasing them, and Thalia makes her final stand.
In the present, Annabeth shares her entire story with Percy and expresses her worry that something might happen to him like it did to Thalia. Percy reassures her that they can't outsmart fate but can do their best and hope for the best. Later, Clarisse informs her soldiers that they will undergo a trial of choice and face Scylla. She's shaken when she sees how dedicated they are to the mission. She and Percy discuss the prophecy the oracle told her, assuring each other of their commitment to complete the quest.
In no time, the vessel reaches the monstrous sea where the crew prepares to confront Scylla. As Scylla seizes three soldiers, Clarisse is overcome by a sense of guilt, causing her to swerve the ship towards Charybdis instead. While she blasts the beast with her cannons, Percy strives to turn the tide in their favor. Annabeth attempts to harness the thermos of winds but finds herself falling. The thermos goes haywire, and the ship begins to buckle under the immense power Tyson unleashes into the engines. The situation turns chaotic, and ultimately, the ship shatters apart, being drawn into the vortex of Charybdis.
