There is a very deadly poison inside that helmet. Aye! If Sunny doesn't receive and antidote within the hour, she will perish(Snicket 197)
When the bautilaires comebackto the submarine from there searching in the ocean, they discover a spore of the mushroom, Medusoid Mycelium, has infiltrated Sunny's helmet and she is trapped within it.
Stay with him if you must, but let us go.(Snicket 291)
Fiona stops Klaus from opening the helmet, as in order to keep themselves from being infected themselves, for the moment they must keep Sunny quarantined in her suit. Fiona comes up with a plan to help Sunny.
Propels the Action By..
Fiona begs her brother to help them get back to the Queequeg, to cure Sunny. Risking his life, he agrees. All 4 of them look for a cure. They are all out of horse radish, the cure. Sunny, using her last breath, lets them know that they can use wasabi instead of horse radish
Fiona trades sides and joins count Olaf with her brother. She can't loose any more family so she feels like she has no other choice but to go to count Olafs side.
With fiona letting the baudelaires go she allows the Baudelaires free access to the Queequeg to escape. But she cannot go with them, out of loyalty to her brother. Before she rejoins Olaf, however, she kisses Klaus. Then they head to there escape.
I could turn my back, instead of guarding you. (Snicket 291)
Because of Fiona the Baudelaires are going to the last safe place to meet the VFD. Violet powers up the Queequeg, as Klaus navigates her out of the Carmelita. Sunny, using the chewing gum Phil brought with him from Paltryville repairs a porthole Olaf broke, and the Queequeg is freed from Olaf's clutches.