Monster Mission (ISBN 9780525464846, 1999) is a children's book written by Eva Ibbotson. It was also published under the title Island of the Aunts. It was a Publishers Weekly bestseller and a School Library Journal Best Book of 2000.
An uncharted island is home to six humans - widower Captain Harper, an old man at the ripe age of one hundred and three; Sybil, a strange cousin who supposedly can predict the future; a chef named Art who claims he committed murder though he can't bring himself to hurt anything living or dead; and the three Aunts. Etta is the eldest and can be a bit intimidating. Coral is a very artistic woman who gathers shells and dances tango by the light of the full moon. Myrtle is the youngest and the most sensitive. She keeps orphaned ducklings in her socks and plays the cello to a selkie named Herbert, with whom she is in love. A fourth sister, Dorothy, is currently in jail in Hong Kong for "hitting a restaurant owner with his own wok", and the fifth sister, Betty, left the island to live an ordinary boring life and raise two overly sensitive children nicknamed "Boo-Boo" and "Little One".
The three sisters take care of injured and sick creatures, but the work is getting too much for them as they get older. They decide to go to London to "choose" (kidnap) children to help them. Etta kidnaps a young girl named Minette, whose constantly bickering parents are separated. Coral brings a boy named Fabio, originally from Brazil, where he is learning to be an "English gentleman" at the horrible Graymarsh Towers. Myrtle is forced to bring a boy named Lambert, whom she thinks is horrid, after he accidentally sniffs chloroform. When Dorothy is released from prison, Betty sends the spoiled Boo-Boo and Little One with her to the island to be looked after when she breaks her hip.
On the island, Minette and Fabio are quickly put to work, including carrying stranded jellyfish back to the sea and holding an eel with scabies. Meanwhile, Lambert is kept in his room because he refuses to help. One day, though, Etta introduces them to a small family of mermaids, part of the menagerie of exotic creatures who sought refuge on the island. The children also meet the Stoorworm, a wingless Icelandic dragon, the egg-bound boobrie (a bird apparently similar to the dodo, but vastly larger) and even talking with the selkies (seals) who can change into humans (they were told if you stab a selkie with a knife it will turn into a human.) Lambert is shocked at the discovery, but Fabio tells them they are hallucinations caused by drugs put in their food. This keeps Lambert quiet, but more determined to be rescued.
After some time, the Great Kraken begins swimming the seas to bring peace to the waters once more. Initially accompanied with his child, he leaves it with the Aunts because it is too young to travel the world with its father. The little Kraken instantly misses his father but quickly befriends Minette and Fabio. Back in London, word spreads about the two "kidnappers". Minette's parents have a "war" as they try to outdo each other's "sorrow for their loss" in the news and Fabio's strict grandparents consider suing the police for not doing their job.
Lambert, though, finally gets a hold of his mobilephone, which he uses to call his father, Mr. Sprott, for help. When Sprott reaches his son and sees all the fantastical creatures on the Island as a business opportunity, he captures them. He reports the island's location to the police who immediately fly off to rescue Minette and Fabio. The two quickly come up with an idea and lead the police to believe Boo-Boo and Little One are the aunt's victims. They are quickly flown back to London and leave the real children free to attempt to rescue their friends, though eventually the Kraken returns and overpowers Sprott's yacht just when all hope is lost. Everyone is rescued, though Sprott and Lambert believe everything that happened was all an hallucination. The Kraken blesses the island and chooses to bring his son with him on his journey. With the "kidnappers" finally revealed, the aunts are put on trial. Minette and Fabio, however, present an argument that convinces the jury that they are innocent. Fabio is allowed to return home to Brazil, and Minette's parents call a truce. The aunts write a will, leaving the island to both Minette and Fabio, who promise to return one day.