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Fred and Pete find a heavy metal thing buried in the sand on the beach. It’s got little windows in, and as Fred discovers, it’s full of gunky seaweed. But what is it? It’s time to head back to the museum to see what Suzy and Archie make of it. Archie reveals that this object will ‘keep you safe and sound’, while Suzy finds a photo of a friend of hers with a big metal object just like the one Fred and Pete found. She sends them back to the beach to meet him. Fred and Pete arrive on the beach and are startled to see what looks like an alien with a metal object just like theirs walking up to them. The ‘alien’ is actually Suzy's friend Iain, and he explains that the metal object on his head is a helmet for a very old-fashioned deep sea diving suit.

Fred and Pete then wonder what divers do when they’re underwater. Suzy sends them to the other end of the beach to meet Hannah from the Cornish Diving Centre, who tells them about modern diving equipment and how they use it. Hannah invites Fred and Pete to take part in one of their dives. The only problem is that the old-fashioned suit is so heavy that he can barely move. Pete tries everything to get Fred moving, but in the end the boys miss the dive altogether. Luckily, Hannah brings them back a silver coin as a small memento to take back to the museum. Finally, Fred and Pete meet up with Suzy and Archie to display their diving helmet treasure in the museum.