Nancy Drew gets a troubling message from her old friend Becca. As assistant cruise director on an Alaskan liner called the Arctic Star, Becca has been making preparations for the ship’s maiden voyage. But someone has been sending Becca threatening emails. The mysterious bully also sent warnings to the celebrity Becca booked for the cruise, causing him to cancel at the last minute. Becca is convinced someone is trying to sabotage the trip. She asks Nancy and her girlfriends George and Bess to come on the cruise and find out who is to blame. Bess’ new boyfriend, Alan, also comes along. Since the girls don’t know him all that well, they keep their case a secret and tell Alan they won the cruise in a contest.
As the cruise begins, an alarmed passenger screams about a bloody body in the swimming pool. It turns out to be a mannequin and cherry-powdered drink mix, but someone is clearly trying to spook the passengers. Nancy and her friends remain on high alert as they get to know some of the other passengers. There’s Wendy, the eclectic-dressing blogger; Fred, who always wears Hawaiian shirts and turns up in unexpected places; Max, the eager butler, who rushes in to help when Nancy’s luggage goes missing; and Tobias, an angry 8-year-old who has sneaked his pet tarantula aboard. Nancy and friends also meet newlyweds Lacey and Vince, who are convinced the cruise is cursed, three elderly frequent cruisers they call the ABCs, the ship’s oft-disappearing videographer named Baraz and a large family of redheads who are having a reunion.
Nancy gets a threatening note in her luggage. Tobias’ tarantula shows up in the dining hall, and a moose antler from the mini-golf area falls and nearly crushes Nancy. She keeps most of the people she’s met on her suspect list, also adding several crew members. Tatjana, the Russian girl working for Becca, is unfriendly. Hiro, the children’s guide, is around when many of the strange incidents occur. And Nancy catches Scott, the activities director, paying someone a large sum of money just before she’s pushed into icy waters in the port of Ketchikan.
Nancy finally realizes Vince and Lacey, posing as honeymooners, are actually working for a rival cruise line and are trying to discredit Becca’s company. The pair confesses to some of the strange happenings and is taken into custody. But there are still a number of loose ends that leave Nancy convinced someone else aboard is bent on making trouble. (In this way, the ending hints at a continuation in book two.)