The Ingo tetralogy is a series of four children's novels, set in Cornwall, by British author Helen Dunmore.
Many people had different theories about how her father disappeared, how he had run away with another woman, faked his own death or had died.
Time runs differently in Ingo and when she finally resurfaces she realizes she has been gone for two days when she sees Conor waiting up for her on the cliff having slept there for the night.
Time goes by and Sapphire is affected by Ingo more and more; craving salt, hearing her name called by the sea and finding it hard to breathe once she resurfaces from the water.
The only thing that can stop her from her yearning from Ingo is Granny Carne (a wizened old woman whom the village folk go to with their troubles).
Roger plans on taking a dive at the Bawns which is the sacred resting place of the dead in Ingo: Limina.
Conor adapts easily to the new city life whilst Sapphire is becoming more withdrawn and turning to Ingo.
She follows it and finds her father, now one of the mer, in a river who had traveled all the way just to warn her that the tide knot is loose.
Just before they leave they learn of the Tide Knot, which has become loose, and shortly after they arrive home it loosens completely; unleashing a terrible tidal wave that floods the town.
Sapphire finally puts him back to sleep using the rowan berries which Granny Carne had given her before she went down to face the kraken.
It received reviews from the Liverpool Echo and The Times,[11][12] and was longlisted for the 2008 Booktrust Teenage Prize.
They meet up with Faro who tells them that Evrys is adamant that they don't accept the call and that he has many followers among the mer who will cause trouble and stop them from making the crossing whether they’re killed or not .
Sapphire's mother and Roger are in Australia for 3 months leaving them home alone with Rainbow and her brother checking up on them every so often.
They join up with Faro after a little trouble getting past the cove which Evrys and his followers had turned into a Porth Cas keeping them out.
The three then go to the assembly chambers and hide from the Evrys followers till they are either accepted or rejected to make the crossing by the mer.
They meet again with Faro and Elvira outside deciding on the best plan of action so they would not be stopped by Evrys' followers.
Conor, Elvira, Sapphire and Faro decide to head North instead of South (the mer's traditional route).
So they head South taking the currents until they’re under the ice of the north where they separate; Conor with Elvira and Sapphire with Faro.
Sapphire and Faro finally meet up with Conor and Elvira they head South again as they travel around the world.
They meet a strange group of mer who have short hair and keep to kelp forests.