Mystery is one of the major three genres of Desperate Housewives.
Each season of the show features at least one mystery which focuses on at least one particular character. The mysteries are always introduced at the premiere of each season, and the story typically concludes in the season finales, although in some seasons the mystery is revealed before the conclusion of said story, as opposed to being solved in the season's climax.
Below is a brief description of every single mysterious storyline, divided into seasons, with the characters focused on headlining the description.
The matriarch of Wisteria Lane, Mary Alice Young, a typical suburban housewife and mother who was calm, gentle and warm, took her own life one beautiful afternoon, which shocked the seemingly pacific community. Her closest friends, Susan Mayer, Bree Van de Kamp, Lynette Scavo and Gabrielle Solis, were horrified to learn their good friend and neighbor had gone off the deep end with no previous omen, and they came across a mysterious note sent to Mary Alice the day she died, which led them to speculate that her suicide was influenced by gruesome factors. In the meantime, Mary Alice's husband, Paul, tried his best to keep his family's secrets hidden, whereas his son, Zach, tried to figure them out.
It should also be noted that a new neighbor, plumber Mike Delfino, moved to the lane to find his missing girlfriend, a story that would tie in with the Youngs'.
Betty Applewhite, a concert pianist, moved to the lane in the middle of the night, along with her teenage son, Matthew. The Applewhites tried their best to keep up appearances, posing as the perfect new family living next door, all the while keeping an extra family member locked up in the basement. However, things started going sour when said family member grew tired of staying in the basement, when Matthew started dating Danielle Van de Kamp, and when the housewives grew suspicious of Betty's constant seclusion and elusive behavior.
Orson Hodge, a dentist with a shady past, eloped with Bree Van de Kamp, and as soon as their engagement was announced to the public, clouds of suspicion started soaring over Orson. Carolyn Bigsby, his former neighbor, shared her opinion that Orson had murdered his first wife, and a cadaver was unearthed at the local country club, which was then linked to plumber Mike, who was suffering from amnesia due to being run over by Orson, who was trying to conceal his secrets and worried that Mike would recognize him from a crime scene. Orson's cold and scheming mother, Gloria Hodge, was introduced to his new family, and tried her best to make sure her son honored the vows he had made to his first wife before their God.
It should also be noted that the mystery was shortened this season, being resolved in episode 3.15, and later on a smaller arc was introduced, towards the end of the season, to serve as a sort of mini-mystery to keep viewers enticed, revolving around Karen McCluskey's disposal of her deceased husband.
Katherine Mayfair, a former Wisteria Lane resident, had left the street in a hurry without much of an announcement beforehand, which left her neighbors and friends puzzled. 12 years later, she returned to the street, along with a younger, doctor husband, and a sweet teenage daughter. As passive-agressive Katherine tries to hide the dark truths the walls of her house once witnessed, her neighbors start to wise up to the fact that something eery is going on. When Adam leaves and Katherine's abusive ex-husband, Wayne Davis, shows up and tries to reconnect with Dylan, Katherine knows she's in trouble.
Edie Britt, who was generally regarded as the local slut, known for her numerous conquests and for typically seducing other women's exes, was kicked out of her neighbors' clique once they found out she'd blackmailed Bree. Immediately afterwards, Edie moved away from the lane, only to come back five years later with a new husband in tow, Dave Williams. At first glance, Dave, whilst perhaps rather peculiar, appears nonetheless to be the perfect new neighbor, kind, caring and friendly. But beneath this surface lies immense rage and a thirst for revenge, one that can't be quenched until someone is dead...
Angie Bolen has spent her whole life running from something or someone, and the latest relocation put her in the suburbs, where she and her family thought they could finally get some peace... after changing their names yet again. A former eco-terrorist, Angie is on the lam, along with her husband and teenage son, from the feds, but also from her pyschotic ex-boyfriend, who is somewhat resposible for her past mistakes.
It should also be noted that a parallel mystery was introduced this season, in a rather unprecedented move, which started with the mysterious strangling of Julie Mayer, who was just one of many victims of the Fairview Strangler...
After 10 years spent in prison, Paul Young returned to Wisteria Lane, and was soon joined by a new wife. His old neighbors were surprised to find that he'd returned, after all the wrong-doings he was behind during his time on the street, both before and after his first wife's suicide. But they weren't so surprised when he finally made his true intentions clear, after purchasing several houses on the lane in a rather complex plot, hoping to pit neighbor against neighbor and friend agaisnt friend, as a means of revenge on all the people who discarded him when he was arrested. Meanwhile, his arch-nemesis, Felicia Tilman, was incarcerated for a short period of time, following the revelation that she faked her death. Which didn't stop her from still carrying out her vendetta against Paul.
Following the accidental murder of Gabrielle's evil stepfather, her closest friends helped cover up what happened, by burying the body in the woods and keeping the secret together, which took a toll on each of their lives. The toll was enlarged when Bree received a mysterious note, very similar to the one Mary Alice got before her death, that appeared to blackmail her. From then on, the women and Carlos, the perpetrator of the manslaughter, have been driven to the brink of insanity, with their lives being continually conditioned by an overwhelming secret and a mysterious creep who appears to keep toying with them. They all share an immense secret, but this doesn't stop them from keeping other secrets from each other, something that drives a wedge between their friendship.