A series of calamitous mishaps beset the girls as they attempt to perform a play about the founding of Heartlake City.

A mysterious blackout causes chaos to distract the city from an evil plan set into motion by real-estate tycoon Carter Greene.

While out horseriding, Mia finds a treasure map and goes to find the treasure. Meanwhile, Ricky and Joey are searching Heartlake City for the treasure map of the explorers on Carter Greene's orders, despite Dr Alvah insisting no such map exists.

A lake monster is scaring all the swimmers out of Lake Heart. Carter Greene wants to search the lake because he's convinced the explorers' treasure is there and doesn't want anyone to see him find it.

Having failed to scare all the swimmers away from Lake Heart, Carter Greene donates a community garden to the city and has Dr Alvah drain the lake so Ricky and Joey can search it while the citizens of the city are distracted by the garden.

Carter Greene needs the Heart of the Sea paintings to check them if they have any clues to the sunken ship. but Ricky and Joey fail to steal both paintings, requiring him to go with them on their next theft attempt.

Andrea accidentally loses her family's historic heirloom locket and tries to conceal it's absence, unknowing that Carter Greene stole it from her.

A television producer arrives in Heartlake City and starts making a documentary about the city. Suspicions are raised when he tells each of the girls separately to keep their role in the documentary a secret from the others.

Carter Greene has Dr Alvah evacuate everyone from City Hall so Joey and Ricky can search the Archives room for maps showing the location of the treasure he wants. When Stephanie brings the girls back to City Hall for her hockey stick, they discover his plan.

Stephanie is feeling diminished by the presence of her older sister Roxy, who has the lead role in the Heartmore musical. Carter Greene becomes interested in the play when Joey and Ricky point out a distinctive brooch used in Roxy's stage costume.

The girls take scuba diving lessons so they are prepared to find the treasure underwater. Carter Greene is getting more insistent that they find the treasure before the girls do.

Emma figures out the engravings they found are a map guide, so they go to get the final engraving to complete the design. Frustrated with Carter's ineptitude, Dr Alvah plans to use her technological inventions to spy on the girls.

Now that both the girls and Carter know the location of the treasure, it is a race to see who can get to it first and claim the prize.

Andrea is experiencing performance anxiety about winning the upcoming talent show. When Vicky reveals a surprising talent for singing and dancing, Andrea's fears about losing the contest intensify and she develops a severe insecurity complex.

Liz has finally completed her first test version of her animal translation program, but when her second test goes completely wrong, she starts to lose her confidence in her skills as an inventor.

When Donna is visiting some sick horses on the other side of the city, the girls are left in charge of the Rescue Clinic overnight. Emma finds an old book which describes a sea monster and reads it to the girls, as strange earthquakes keep shaking the Lighthouse.

Barney has failed his police horse physical aptitude test several times, so Olivia volunteers the girls to train him to pass the next test and stay on the force with Laura.

Dr Alvah becomes very worried that Zobo will help the girls foil Carter's new plan, so she tries to get rid of him by hacking into his computer core in order to take control of him and make him highly destructive.

When Emma doesn't refill the cruiser's fuel tanks before they go out to sea, the girls end up stuck in the middle of the bay when the cruiser uses up all it's remaining fuel.

While attempting to take their painting to the Explorers' Cave, the girls discover that Carter's underwater excavation project is becoming dangerous. Meanwhile, Dr Alvah is starting to discover that Carter cannot be trusted.

When Andrea loses her voice for a day due to singing too much, she becomes despondent. As a show of support, the other girls volunteer to give up something for a day as well, but discover keeping that resolution is not easy.

The girls are starting to investigate Carter's undersea construction zone, but the arrival of a new girl in town disrupts their investigation in more ways than one.

When the girls start investigating Carter's personal history, they discover some interesting new facts that causes Kacey to doubt her loyalty towards her employer.

When the girls discover the boardwalk is damaged at the exact time a charity concert is being held there, they have to figure out how to repair the support pillars enough to keep the platform from falling into the ocean.

The girls have finally lost patience with Carter and plan to end his dangerous construction scheme forever.

With the construction project halted, Donna and the girls can return the sanctuary animals to the ocean. Stephanie has an idea, get the bay declared a nature sanctuary so no-one can ever build in the water again.