|Original title||Farlig midsommar|
|Cover artist||Tove Jansson|
|Preceded by||The Book about Moomin, Mymble and Little My|
|Followed by||Moominland Midwinter|
Moominsummer Madness (Swedish: Farlig midsommar Literally: Dangerous Midsummer) is the sixth in the series of Tove Jansson's Moomin books and the fifth novel, published in 1954. The story follows the Moomin family as they find themselves living in a deserted theater after a flood.
Moominsummer Madness was inspired by Tove Jansson's work with theater, it is dedicated to Vivica, which most likely refers to her friend Vivica Bandler, who directed the two original Moomin stage plays and the 1969 television series.
A nearby volcano causes a massive wave to flood Moominvalley. While escaping the flood the Moomin family and their friends find a building floating past, and take up residence there. They believe it is a deserted house until they realise someone else lives there, Emma, who explains that it is not a house but a theatre. The moomins start to understand about the scenery, props, and costumes they have found. The theatre drifts aground and Moomintroll and the Snork Maiden decide to go and sleep in a tree.
When they wake next morning the theatre has floated away again and they are alone. Meanwhile Little My accidentally falls overboard, and by some strange coincidence is rescued by Moomintroll's adventurous friend Snufkin who is setting off to seek revenge on a grumpy Park Keeper. He tears down all the 'Do not walk on the grass' notices, fills the lawns with electric Hattifatteners and sets free twenty-four small woodies who immediately adopt him as their father.
The coincidences continue as Moomintroll and the Snork Maiden meet Emma's deceased husbands niece, the Fillyjonk, and all three get arrested burning the signs that Snufkin tore up. Meanwhile in the theatre, Emma helps Moominpappa write a play and the family decide to stage it. The woodies find a playbill for the play and cajole Snufkin into taking them to the theatre. The Hemulen who has arrested Fillyjonk, Moomintroll and the Snork Maiden also finds a playbill and leaves his cousin to guard the prisoners while he heads off to see the play. The cousin is persuaded of their innocence and lets them out to go to the play too, where everyone is reunited and ends up on stage, the play itself collapsing into a big reunion party. When the floods recede everyone gets to go home.
Moominsummer Madness has been adapted to television four times.
The Polish 1977 version was restored with updated effects and cut into a feature length film in 2008.
The story from Moominsummer Madness was also the basis for the 1958 stage play Troll in the Wings and the 1974 Moomin Opera. As a result of the 1958 play incorporating songs into the shows, there are several Moomin songs based around the characters from this book.