The Grim Grotto recap

Drifting down the Stricken Stream, the Baudelaires are taken aboard the submarine Queequeg, commanded by the blustering Captain Widdershins and crewed by his stepdaughter Fiona, a young mycologist, and Phil, the optimistic cook from the lumbermill. Their mission is to recover the sugar bowl, an object vital to V.F.D., hidden in an underwater cave. Diving into the Gorgonian Grotto, they encounter the deadly Medusoid Mycelium, a poisonous fungus, and Captain Widdershins vanishes. Sunny is poisoned and will die within an hour unless cured; the children discover that horseradish dilutes the fungus and use wasabi to save her. Count Olaf's submarine then captures the crew, and Fiona is reunited with her long-lost brother, who is one of Olaf's henchmen. Fiona helps the Baudelaires escape but chooses to leave with her brother and the villains. At Briny Beach the children meet the V.F.D. ally Kit Snicket, who sends them onward to the last safe place, the Hotel Denouement.

The Baudelaire orphans, Violet, Klaus, and Sunny, lost their parents to a fire and have been hunted ever since by the greedy actor Count Olaf, who pursues their fortune in endless disguises. Over many homes and escapes they befriended the Quagmire triplets, uncovered a secret organization called V.F.D. to which their parents belonged, and were framed by Olaf for murder, becoming fugitives. In the frozen Mortmain Mountains they reunited as a family, met the third Quagmire triplet Quigley, and learned that V.F.D. is split between noble and villainous factions warring over a mysterious object called the sugar bowl, and that both sides are converging on a distant gathering. Now, having drifted downriver after being parted from Quigley, the three siblings continue their search for the truth about V.F.D., the sugar bowl, and whether any of their allies, or a parent, might still be found.

Floating down the Stricken Stream, the Baudelaires are rescued and taken aboard the submarine Queequeg, commanded by the fierce, blustering Captain Widdershins, who lives by the rule that one must never hesitate. His crew includes his stepdaughter Fiona, a bright young expert on mushrooms and fungi, and, to the children's delight, Phil, the optimistic worker they once knew at the lumbermill, now serving as the submarine's cook. The crew is on an urgent mission for V.F.D.: to recover the sugar bowl, an object of great importance to the secret organization, believed to be hidden in an underwater cave. The Baudelaires join the search, glad to be among allies at last, as the submarine sets course through dark waters toward the grotto where the sugar bowl may lie, and toward the dangers waiting there.

The submarine reaches the Gorgonian Grotto, and a diving expedition into the underwater cave brings the children into contact with the Medusoid Mycelium, a deadly poisonous fungus that grows and recedes in cycles. During the mission Captain Widdershins mysteriously disappears, leaving the young crew to fend for themselves. Disaster strikes when Sunny is exposed to the fungus and poisoned; its spores take hold in her, and she will die within an hour unless an antidote can be found. With time running out, Violet, Klaus, and Fiona search desperately for a cure, poring over Fiona's mycological knowledge and the resources aboard the submarine, racing to save the youngest Baudelaire before the deadly fungus claims her life.

The children discover that the deadly fungus can be diluted by horseradish, and, finding wasabi aboard the submarine, they administer it to Sunny and save her life just in time. Their relief is short-lived: Count Olaf's own submarine hunts down and captures the Queequeg's young crew. Aboard, Fiona is reunited with her long-lost brother, only to learn that he is one of Count Olaf's henchmen, which forces her into an agonizing choice between the Baudelaires, who have become her friends, and the only family she has left. As Olaf gloats over his captives and presses his search for the sugar bowl and the deadly fungus, the Baudelaires must find a way to escape the villain's submarine, uncertain whether Fiona will help them or turn against them.

Fiona helps the Baudelaires escape Count Olaf, but she chooses to remain with her newfound brother and join the villains, taking a sample of the deadly Medusoid Mycelium with her, a painful betrayal for the children who had trusted her. Reaching Briny Beach, where their misfortunes first began, the orphans meet Mr. Poe again and are met by Kit Snicket, a member of V.F.D. and the sister of the murdered Jacques Snicket, who is expecting a child. She warns them of a decisive gathering to come and urges them onward to the last safe place, the Hotel Denouement, where the two factions of the secret organization will converge. Having lost yet another friend to Olaf's side, the Baudelaires set out for the hotel, drawing ever closer to the center of the conflict that has consumed their lives.

The Baudelaires save Sunny from the deadly Medusoid Mycelium by diluting the fungus with wasabi, narrowly escaping the poison's grip. Captured by Count Olaf's submarine, they meet Fiona's long-lost brother, who turns out to be one of Olaf's henchmen, and Fiona, torn between her new friends and her only family, ultimately helps the children escape but chooses to leave with her brother and join the villains, taking a sample of the dangerous fungus with her. Reaching Briny Beach, where their story began, the children encounter Mr. Poe once more and are met by Kit Snicket, a member of V.F.D. and sister of the murdered Jacques, who is expecting a child. She warns them of a decisive gathering to come and urges them toward the last safe place, the Hotel Denouement, where the two factions of the secret organization will converge. Having lost yet another friend to Olaf's side, the Baudelaires set out for the hotel, drawing close to the heart of the conflict that has shaped their lives.