Kate's Island is the eighth episode of the LEGO Friends TV Series.
Olivia ropes the Friends into going with her to science camp. There they meet a mysterious girl named Kate. Olivia does not like her at first, but they all later become friends while bunking in their cabins and solving the mystery of the legendary Dark-eyed Kate.
Olivia is excited to be attending science camp. Everyone is on the docks waiting for the ferry to the island. Stephanie is struggling to move one of Olivia's bags and asks her if there are rocks in it, yes, there are, it's Olivia's rock collection.
After they get on the boat to the island where the science camp is located, Olivia is studying some of her rocks. Mia notices a girl standing by herself and asks who she is, but nobody knows her. The girl is holding a metal case and keeps looking at it. One of the boys takes the case from the girl and they start a game of keep away with it before Emma intercepts one of the throws to catch the case. While Emma is admiring the design of the case, the girl takes it back and leaves without speaking. As she does so, she kicks some of Olivia's rocks into the sea, causing Olivia to start hyperventilating and Stephanie has to calm her down. The girl apologises for doing so, she didn't know the rocks were special to Olivia.
The girl introduces herself as Kate, but states the similarity to the name of the island is just co-incedence. She backs away and starts looking at the case again. Mia is curious, but is distracted by Miss Stevens announcing they've arrived at the island. Olivia starts imagining the fun she will have, discovering new rocks for her collection. Andrea's imagination takes her to a different scenario, sunbathing on the beach while eating ice cream.
Mia asks why the island has it's name, Stephanie explains that a long time ago, it was a pirate hideout for Dark Eyed Kate. Unnoticed by the girls, Kate reacts to the explanation. Stephanie adds that there are rumors of a hidden treasure on the island. Miss Stevens says that it's just a story, there is no proof if it is true. Olivia says they're too old to believe in kids stories like that. Kate's reaction is a cynical glance and shaking her head slightly.
At the science camp, Miss Stevens is seperating the campers into two study groups. The first team will be studying the ocean, the second team, geology. Emma doesn't want to the group assignments to seperate the girls, so Stephanie makes sure to move each girl into the lineup so they will be counted as group 2. Unintentionally, Kate is also chosen for group 2. Olivia is excited, Kate is bored because she doesn't think rocks are interesting. Olivia starts talking about geological science, then Kate insists she stop. Olivia is confused about Kate coming if she's not interested in science, Kate says she just isn't interested in geology. Miss Stevens says each team will present their findings in two days, Stephanie gets excited at the thought of a contest and winning it, despite Miss Stevens pointing out it is not a contest.
On the beach, the girls are collecting geological research data and Andrea is looking bored. Mia notices Kate walking off alone. Mia thinks Kate has so no friends, so suggests that they become her friends. Stephanie insists that Olivia focus on the science, since she wants to win the competition. Olivia starts scanning the beach with a metal detector and it starts beeping rapidly, leading her to Kate. The metal detector was reacting to Kate's metal case. When Olivia notices it, Kate hides it from sight and looks guilty before walking away.
Emma looks down at her rock art creation, a large heart pattern. Mia tries to make friends with Kate, who ignores her and runs away into the forest. Mia follows her and isn't noticed until she steps on some sticks. The sound of the sticks breaking causes Kate to look back and Mia dives behind a bush to hide.
Mia keeps following Kate into a crowd near the island lighthouse, but then Kate disappears. Mia looks upwards to see Kate standing at the top level of the lighthouse and looking out over the island while holding her case. She tries again to make friends with Kate, who leaves abruptly. Looking down at Kate, Mia notices the case again.
At night, the girls are collating their geological research, when Olivia asks where Kate's rock samples are. Kate is surprised by the question, distracted by looking at her case. Kate claims she left the rocks on the hill and will go back to get them tomorrow. Mia looks uneasy at the lie Kate is telling. Olivia asks what kind of science Kate likes and is told plants, specifically flowers. Olivia is suspicious and Kate says she needs to sleep.
Later that night, while all the girls are sleeping, Kate sneaks out of the window. The sound of jumping down wakes Mia up, who follows her outside. Kate is annoyed that Mia keeps following her around and Mia asks for an explanation. Kate says that she is looking for Dark Eyed Kate's treasure and asks Mia to keep it a secret. She shows Mia the case, explaining it was given to her by her grandmother who told her it was a clue to where the treasure is. Mia says they will look for the treasure tomorrow. When they return to the cabin, the sound of the two girls climbing back in the window wakes up Olivia, who pretends to be asleep as they walk past.
The next morning, Olivia's group are getting ready to take a flight in a hot air balloon. Olivia says Kate should be excited and uses a lot of long scientific words, all of which confuse Kate. Kate assumes Olivia is talking about geology again, then Olivia explains, she was talking about flowering plants. Olivia starts to question Kate intensely, but Mia distracts her. Olivia makes a "I'm watching you" guesture at Kate before they get into the balloon basket.
While the balloon is flying along, Kate is distracted and keeps looking at the ground below. Andrea notices, telling Kate she finds science boring too, but the view is nice. Olivia is sad that Andrea is not having fun. Kate looks down at the island to see a large X on the penisula below. Kate takes Mia aside and holds up the case, which makes Olivia suspicious about secrets being kept. She tells the girls that they sneaked out of the cabin last night, to which Andrea is disappointed that they didn't invite her to come, because it would have been the only fun she got to have on this trip.
After the balloon flight is finished, Mia and Kate are still talking alone while Olivia's team collates more research data. Emma has been sketching Kate's case, Olivia takes the sketch and compares it to the aerial photo of the island, noticing there's a large X on both. When Mia sees the sketch and island photo together, she gets worried, then goes away when Kate calls her back. Stephanie is getting suspicious, so she says they have to watch the two girls all the time.
Late at night, Kate and Mia are sneaking out of the window. All the girls were pretending to be asleep, except for Andrea, who was really sleeping! Stephanie doesn't want to follow them outside, because getting caught outside will cause them to lose the non-existent competition. Olivia has a solution, she makes a machine that fakes snoring each time someone walks in the room and hides it in their beds.
The girls are walking out of the forest and along the shoreline, Emma and Andrea getting scared of the dark. Then they hear a strange sound behind them, which scares all the girls. They don't want to turn around to see what was following them, then Stephanie says they have to look. The girls realise they were being followed by a friendly seal and start gushing over how cute the animal is.
Kate and Mia are found by the other girls, standing near the rocks and examining the case. Kate is annoyed that they were followed, but then accepts that the others can help. They found some carvings on the rock, but don't know what they mean. Olivia realises they match a constellation used for navigation and realises the brightest star is different from the others. Kate places her case into that carving and there's a click as a hidden lock opens. The girls push the rock and it slides aside, it was really a door to a hidden cave.
Inside the cave, there's a wooden crate on a pedestal. Kate opens the crate and finds Dark Eyed Kate's diary inside, which has a family tree on the middle pages, revealing that Kate is a descendent of the legendary pirate. Olivia is investigating the crate further and there's a lot of rare fossils inside, which gives her the realisation that Dark Eyed Kate was really a scientist. Kate finds it amusing that her ancestor likes science and she does not.
The next morning, Olivia and Kate are giving a presentation about the fossils they found. Stephanie is excited that they won the completion which exists only in her mind.
- The Friends all have new tops, which are their science camp T-shirts.