In the Season 3 finale, "The Toll of the Sea," in a desperate gambit to save Hope, Ryn and Ben (Eline Powell, Alex Roe) confront their nemesis Tia (Tiffany Lonsdale) in a high-stakes undersea battle between the mermaid tribes. Elsewhere, Maddie and Robb (Fola Evans-Akingbola, Deniz Akdeniz) desperately race to find a cure to save Xander (Ian Verdun), while Helen (Rena Owen) and the hybrids try to restore order. Ted (David Cubitt) struggles to accept his son's reality.