Cat has always felt a few steps behind — size-wise, speed-wise and making-friends-wise. Being plonked into a dull mining town, smack in the middle of the school year, doesn't help. But things could be looking up when her swim-crazed neighbours, Keeya, Tumi and Neil, include her in their friendship gang, the Marlins, and get her to join the local swim club, where she grows her confidence and shrinks her times.

One Sunday, the Marlins see a mysterious digger at a deserted house, hiding something. They decide to investigate. Asking around reveals nothing except a suspicious-acting teenager. Video surveillance is their next strategy. While their nanny cam films, they try to figure out who the digger dude is AND keep training and competing in swim meets. They badly want to win that medley relay.

Then things go south — both in the relay and with their spy-bear. Despite the sour vibes, Cat takes a wild risk to show her friends they can trust her again. To gather enough evidence before it's all gone, and to keep them all out of danger while they're out there.