The children of Seacrow Island take a public ferry to visit a relative only to find that she has went to hospital. When they try to row back home they lose the oars and must take refuge on a desolate island. 4-year-old Skrallan is the first to discover that two colorful smugglers are hiding on the island.
Tjorven and the other children at Seacrow island have received a play day in a gift. Uncle Melker will find himself ending up in the lake several times. The kids also have found an old ship that will be named Albertina. They divide themselves into two pirate gangs and fights about the wishing stone Mysak.
Vesterman has found a young seal in his fishing nets in the outskirts of the archipelago. When he comes back to the Saltkråkan island he gives the seal to Tjorven, who names it Moses. Peter Malm, a visitor who works at the Zoological Institute in Uppsala, wants to buy the seal, but Tjorven says it's not for sale. Vesterman is in need of money and tries to get the seal back, to sell it to Peter. The children have to hide the seal, so he won't find it. Pelle's rabbit Jocke and one of Söderman's lambs are found bitten to death. The dog Båtsman is accused of those evil deeds, which means that Tjorven's father has to shoot her dog. In the last minute Söderman finds out that a fox is the perpetrator. Peter Malm says he won't buy the seal. Vesterman is disappointed and the seal stays with the children.
A writer spends the summer holidays with his children on an island. The family grows together through friendships with humans and animals and through adventures.
