Moominsummer Madness (Swedish title Farlig midsommar, or "Dangerous Midsummer") is the fifth in the series of Tove Jansson's Moomins books, published in 1954.

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.

Snufkin 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 Moonmintroll and the Snorkmaiden meet Emma's deceased husband's niece, the Fillyjonk, and all three get arrested burning the signs that Snufkin tore up.

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.